I reckoned that the gear linkage would be pretty straightforward (yes, you’d think I’d know better by now) so I started with the brake.

The problem with the brake linkage is that it’s a good old fashioned drum brake with a rod linkage. Incidentally, it works extremely well, even the MOT tester was surprised. Anyway, the issue with a rod gear linkage is one of matching the geometry of the linkage to that of the swinging arm, so that,
a) it doesn’t bang the brake on at full bump or droop  and,  
b) the brake still works at any suspension position.
I must confess to messing around with bits of welding rod, string and whatever else came to hand for some hours before finally coming to my senses and seeing the blindingly bleedin’ obvious. The original linkage worked perfectly and had all the right bits to connect to the brake light switch.  Once that penny had dropped it was the work of a few minutes to do a couple of quick sums to get the ratio of levers right. I wanted a rather less movement than on the original lever, but not so little as to reduce my ability to brake hard. On measuring the various bits and pieces at my disposal I opted for a pedal that would require one and a half times the pressure of the original (I’d be in a better position to push on it and I reckoned that the bike was designed for small eastern youths, not European adults) but with only a centimetre or less movement. 
Once the sums were out of the way I simply chopped off most of the original brake lever and drilled a hole in the remaining stub to accommodate a straight linkage from the pedal.  No problemo!

The gear linkage would have to wait until I had an engine in the frame, but that would be no problem at all, as the linkage and lever  from the Aprillia would fit, honest.