Two steps forward...

Turns out the only thing I thought I'd done right yesterday was,  in fact,  wrong :-).  Well,  not horrendous but the passenger footwell panel was on the wrong side of the chassis - should have been the engine side. Just means I had some extra holes on the inside,  which actually turned out quite well as it allowed me to attempt my first rivet (as a means of filling in the holes)  - you can see them in this pic:

Other improvements - the latest coat of paint on the caliper has greatly improved things,  although that may be because I went a bit nuts and splodged a thick coat on.

Also,  I finally got the crush tube out of the pedal and started filing it down but it's taking ages and really going through the emery paper:

At least all the interior panels are now drilled and ready to fit.

Called GBS today and they have the diff fitting kit for a measly £14 including delivery,  which is incredibly reasonable (the 2 bolts alone cost £10 undelivered from eBay). On top of that,  I ordered it today and I got a text message from the courier firm to say it would be delivered tomorrow! Very impressed...