Pbehn, did you ever race up in Scotland at all?
Just getting back to aircraft maintenance during wartime, a good team of engineers can do any type of maintenance on an aeroplane anywhere - the big thing being availability of tooling and ground service equipment (GSE, cranes etc). Aircraft maintenance at a well supplied air base, with good workshops and ample space, means that the engineers can do what's required to the airframes, scheduled maintenance or otherwise in-situ, particularly if the base was far from repair depots. Every aircraft has a maintenance schedule based on cycles and flight hours, so inspections were regularly carried out and work arising based on these was also done. Obviously in wartime, unscheduled maintenance, i.e. defect rectification probably took on a priority unexpected in civil times, but it was always intended that regular inspections were expected and schedules kept. Different aeroplanes have different inspection and repair criteria and although these days things are a lot more structrued than they were in wartime, the same degree of planning was required. The big thing being available man hours, facilities and tooling and also space, particularly the latter on a busy airfield; a lot was done under the stars.
The big driver of course is requirement versus availability of aircraft based on their maintenance schedule, so sending airframes away unnecessarily was probably avoided. I'd imagine during the war that busy air stations were chasing their tails with repair work as much as attempting to keep the planned maintenance schedule for each machine. These days most MROs (maintenance, repair, overhaul facilities) have planning and maintenance control departments, but at a wartime airfield, such luxuries were probably not available and decisions as to which airframe gets taken off the line would have been made by the chief engineer. Depending on where the air base was and how close it was to a heavy maintenance base, the aeroplanes would have undergone everything where they were; there would have been no point sending an aeroplane away for maintenance if it could be done in-situ, like rebuilding flight controls, scab patches, even heavy maintenance (major structural inspections, in the airline industry C and D checks) etc, but again, this depended on the facilities available at the air base. Without good workshops and tooling etc, airframes and parts would have to be sent away.
In my job we have a team of guys who travel around the country doing maintenance at poky little airports around the place, but parts have to be sent with them and again, what is done is restricted by what GSE is available. Obviously engineers take their own tools with them, socket sets, screw drivers, ratchets, spanners etc, but if something needs to be done where a hydraulic line is opened and there's no rig to do pressure and integrity checks, the work is delayed until the airframe gets to home base and it's put under dispensation of sorts - in the airline industry this is called MEL, which is divided into categories depending on the severity of the defect and the system affected. For example, if you have a de-ice boot that doesn't inflate, the aeroplane cant fly in rain. If its a cargo bay light, an MEL item, then a bulb change has to be done within a period of time, but it won't ground the aeroplane. If its more serious, pax cannot be carried and a ferry flight is carried out to get the aircraft home as soon as possible. Obviously requirements during the war would have differed, but getting teams of guys to isolated places would have happened.