Firstly, thanks to all the people on this thread who have contributed to fixing my aileron range problem, and also the interesting background information on aileron rigging!
It turns out that, unbeknown to me, the aircraft design *does* have 'primary' aileron stops which were supposed to be fitted to the kit by the factory; I was not supplied with the parts (or instructions) to build them, anyway.
Longer story is I went to a local maintenance facility to look at a factory-built model, and there were the stops -- hidden discretely underneath an aileron hinge - they act on the 'leading edge' of the aileron when it is in its fully 'down' position.
When I receive the parts I've ordered, fixing the problem should be simple: Commensurate movement (more up than down) is fundamentally down to the geometry of the bellcranks
Thanks again all