i found this on the Net, from a guy called Bob Best. Its a good read, if nothing else...
"It is often stated that the Spitfire and Hurricane could both out-turn the Bf109. In his memoirs, Spitfire pilot Alan Deere claimed that it was impossible for a Bf109 to follow him in a turn, and that a hard defensive turn would quickly turn the tables on an attacking German. However, some sources claim the exact opposite, and some - but not all - German pilots also state that theirs was the better turning mount. Who, then, is right? How much difference did the aircraft flown make to the outcome of the Battle of Britain?
In making a comparison, there are several variables to consider. Altitude is one variable, as the German machine is held to be superior at higher altitudes by virtually all sources. Pilot quality is another factor, and it is this which may have made the Messerschmitt seem less maneuverable in combat for a couple of reasons.
First, many of its pilots believed it was not structurally sound. Its wings were both small and thin - so thin that prominent bulges were needed to house the breeches of its wing-mounted guns. The undercarriage was mounted on the fuselage rather than on the wing spar, the latter not being strong enough to withstand the impact of landing, which also gave it a dangerously narrow undercarriage track. The tailplane had bracing struts to cure structural problems resulting from engine vibration. The German machine looked light and dainty and its pilots simply did not trust its strength, although it was more than adequate given the light weight of the aircraft as a whole. In fact the whole design philosophy of the machine had been to make the wings as small and thin as possible to give maximum speed and maneuverability, which also dictated the overall small size of the plane to keep its weight to a minimum.
Second, when it was approaching its limits of low-speed handling, its leading edge slats automatically deployed, with a loud bang clearly audible over the engine roar! Although still having a considerable margin before stalling once the slats deployed, most pilots would back off their turn at that point, believing they were right on the verge of stalling. Many others did not even reach this point, and used the point of the slats deploying as the limit of safe performance, flying their aircraft in order to prevent their deployment at all. In this way these slats hindered their machines maneuverability, rather than being used to enhance it as was intended. This was a deficiency of the pilots, rather than of the designers, but when flown without the assistance of the slats the Bf109 would certainly be less maneuverable than its opponents.
An opposite fallacy exists with regard to the Bf109's ability to escape from the Spitfire by diving away. Early in the Battle, Spitfire pilots gave up the chase as considerable distance would be lost initially by the Spitfire having to roll before diving to prevent engine cut-out. However, as some pilots, through ignorance or frustration, followed they found that the 109 would have to pull up sooner or later, and when they did pull up it was done gently to avoid overstressing the thin wings, which was a genuine weakness of the design. The Spitfire pilots had no such fear, as their larger, thicker wings had a hefty "leaf-spring" spar which conferred enormous strength, and could simply "cut the corner" when pulling up their nose to make up their lost ground. Furthermore, diving to a lower altitude played to a strength of the Spitifre, as its performance was better at low altitude than high altitude, where the 109 had the advantages.
This is itself a factor to be considered in this comparison was the captured Bf109 that the British tested. They did not have an oxygen bottle for their captured Bf109, so tested it at only low to medium altitudes, where they thought combat would take place anyway. At these altitudes the result was indeed that both the Spitfire and Hurricane could out-turn the Bf109, and this was reported to the squadrons, whose pilots would have reacted in combat according to this perceived strength. Later, well after the Battle was over, testing at higher, "combat" altitudes showed the opposite to be true at these heights. Yet pilots in the Battle had consistently confirmed what the initial testing had told them, and claimed they were able to out-turn the German machine at all altitudes!
Finally, in assessing the merits of the machines, there is one final variable - the improvements made to the Spitfire and Hurricane between May and October, or more precisely, to their powerplant. Both started their service careers with two-blade fixed-pitch propellors being turned by an engine running on 87-octane fuel. In 1938 production of both types switched to a three-blade two-pitch propellor. To be more precise, this meant the engine was changed to a new version of the Merlin which could accomodate the new propellor, and this changeover caused some production delays. Since supplies of the newer engine were scarce, rather than convert earlier examples many of them were withdrawn from front-line service and placed in Operational Training Units, where their performance was not critical but they still served a more than useful function. With the Hurricane, however, sheer weight of numbers produced meant many two-bladers stayed in front-line squadrons, and numerous examples served in the battle for France. In fact, the majority in France were two-bladers, and having less-than-sparkling performance, these may have had something to do with the German assessment of the Hurricane as being markedly inferior. It was, but the margin was not as great as they thought.
The next change was to upgrade the propellor from a two-pitch to a variable-pitch. This change started only in early 1940 on the production lines, but there was no program put in place to upgrade existing examples. It was only when Geoffrey de Havilland sent teams out to the squadrons on his own initiative - and in hope he would later be paid for the work - that the older aircraft were all updated. These mobile conversion teams would arrive at a squadron, fit one propellor in front of the local fitters, assist with the conversion of a second, then just supervise the third. This done, enough kits were left for the whole squadron and they were on their way. This program started in July and was finished by mid-August.
The final upgrade was to the petrol. In 1940, producing 100 octane fuel was a complex process and the plant to produce it on a commercial scale did not exist in Britain. However, supplies had been imported from overseas and stockpiled from well before the war. In May 1940, these supplies were released to squadrons and the effect was startling to the pilots, as their Merlins went from 1,030 to 1,310 horsepower. This gave marked improvements to acceleration and climb rate, although the extra weight of the armour and radios then being fitted offset this slightly.
The net effect of these incremental changes was that performance of the British machines varied wildly. Within six months, the Luftwaffe went from facing the Hurricane Ia with a two-bladed wooden propellor and a Merlin II engine producing 990 hp, to facing the new Spitfire IIa with a constant-speed propellor and a 1,175 hp Merlin XII engine, while their own Bf109E's performance remained constant. Add to this that even similar aircraft vary in performance from example to example, and it is no wonder that performance comparisons are so difficult and opinions are so varied!
Finally, how an aircraft is flown in combat will differ from the way it is flown in clinical trials. This could also have resulted in a self-fulfilling prophecy. Perhaps, having been told their mounts turned tighter as a results of their own testing, RAF pilots always tried to turn inside their opponents. If they were the better pilots or if conditions were favourable they may have done so, scored a kill and lived to tell the tale of how they did it, perpetuating the myth. If not....
What can be said with certainty is this: the performances of Bf109 and Spitfire were very close, with any edge either held in any area being only minor. Even the Hurricane only lagged behind its contemporaries in speed, but compensated with far greater resilience. Man, not machine, would have made the major difference in clashes between these aircraft."