For example with the UK2000 scenery for Heathrow it's telling me that stand 508 isn't large enough for a 767-300, when I can park there with plenty of space at either side and it's used for 763s in the real world
That doesn't happen here. GSX at UK2000 doesn't label the Gate 508 as being to small, tried with the Level-D 767-300.
You taken a very borderline example, since in the AFCAD I have (coming from the Trial version for UK2000 EGLL, named egll_AFX_UK2xTRM.bgl), Gate 508 is labeled as "Small" with a radius of 24 mt. The Level-D 767-300 wingspan divided by 2 is 23.7870999492 mt so, if you are using another 767-300 model that has been defined a bit larger than 24 meters, or you modified the AFCAD to set, let's say, 23 meters, GSX will say the parking is too small.
I'm very impressed with the program as a whole and can see that a lot of effort has been put into it, but it'd be nice if this gremlin could be sorted.
There's no "gremlin" and nothing to be fixed in GSX. You are asking for a different thing, which is allowing GSX to just ignore the parking size, on the assumption it might have been set too small by mistake.
But, of course, GSX can't possibly know if that was really a mistake, and there's plenty of space at such parking, or it IS really a parking too small, GSX really need to trust what's inside the AFCAD, and if the AFCAD it's wrong, it's the AFCAD that must be fixed.
Try to post the question in a different way: why an airport designer specified a parking radius smaller than it really is ? It's possible there are sceneries around from pre-FSX times, when it was difficult to assign airlines to parkings, and some designers tweaked the parking radius to direct airline assignment, and there might be AI models with non realistic parking radius set on purpose to allow this technique, but is something that shouldn't be done in FSX anymore.
The parking radius is used also to place vehicles and calculate their approach paths to the airplane, if the parking is REALLY cramped, if we allowed vehicles meant for larger airplanes (the larger catering and pushback vehicles, that are used for larger airplanes, for example, can't turn very well in a small parking) in a parking that we don't KNOW it's really larger than what the AFCAD is saying, all sort of troubles will start, and users will of course blame GSX, when the problem was clearly the AFCAD.
So, we might add an option to set a different parking radius that GSX will use to accept the airplane, but it can't be a global option, it must be something related to customizing an airport, so you can set a larger radius only after you have checked it IS larger indeed.