Hmmm bloody car...
Unseizing the calipers did little to increase the brake effort. Turns out the rear brake compensator has seized. So me and my Dad made up some new pipes to bypass the compensator just to get it through the MOT, and replaced a few of the brake pipes with the copper pipe we had left.
Went to bleed the brakes out afterwards and now the 'kin master cylinder is shot

Never heard my Dad swear so much
Good job we've got the trusty old Triumph as a back-up car
(On the plus side I've butchered the Mi air filter housing in a moment of madness and I'm loving the induction noise

)