Ok, so I'm a really sad git with no friends who ends up being the only person who responds to his own posts....
Anyway, a closer inspection of Warner's "Bristol Blenheim - a Complete History (2nd Edn)" reveals some interesting info on p.101 which, to my simple mind, merited my dusting off this old (and completely ignored) thread. According to Warner:
"The Blenheim was easily adapted to this unexpected new [fighter] role by fitting an external pack containing four fixed Browning .303 machine-guns beneath the bomb bay, mounted on a sub-frame of mild-steel tubing within the bay, which also contained 2,000 rounds of ammunition - 500 rounds for each gun, giving more than 20 seconds of continuous firing. The total weight of this armament was only slightly less than the normal bomb load. This package was first flight-tested at the A&AEE on L1424....and improvements to the ammunition feeds were made, together with provision for heating to prevent the gun-oil from freezing at altitude. Manufacture of these gun packs and their mounting frames was sub-contracted to the Southern Railway workshops at Ashford in Kent, which produced more than 1,300 sets altogether...The single fixed Browning in the port wing was retained, as was the dorsal turret with a single Vickers machine-gun."
Given the above description, one must assume that the central bomb-bay doors were removed to enable fitment of the gun pack onto the existing hard-points under the wing centre-section. I also presume the gun pack installation came with blanking plates fore and aft of the pack itself to cover the gap resulting from removal of the central bomb-bay doors (ie to improve streamline and prevent dirt being thrown up into the bomb-bay). Does this make sense? I'm just struggling to see how a tubular structure strong enough to carry almost the equivalent weight of the Blenheim bomb load, could be inserted and still keep the central bomb-bay doors in place.
Other thoughts/ideas would be MOST welcome. Please talk to me before I curl up in a foetal position and start rocking myself while muttering incomprehensible musings on the nature of gun pack installations.