Thanks fella.
I'm now certain it's the thread lock. I'd read a few other MadCap project/build threads elsewhere where people reported certain screws (self tap into plastic) working loose & to recommend using thread lock. This has cost me dearly as every plastic part which has been in contact with thread lock has reacted with it & cracked, this includes the chassis, rear hinge pin blocks & rear shock tower mount. These parts I had spare luckily but they're getting quite rare so it's annoying to have used up the spares just because of thread lock
On a (hopefully) positive note, the slightly slipping diff (for which I had no more 0.05mm shims) is now sorted. I managed to locate some 0.20mm 'washers' I thought I had 2 x 0.05mm shims in each side of the diff but there were 3
So I've removed those, fitted the 0.20mm shims & now it feels nice, plus I now have 6 x spare 0.05mm shims for if it starts to slip.
One thing which did happen was after running the car for 2-3mins I noticed one front & one rear hinge pin had unscrewed themselves
so I need to either fit proper hinge pins/e-clips or just hope it doesn't happen again...
Just need to get the shell sprayed now (hence no more pics yet) then find an opportunity to try it at a track.