The handbrake has never worked well on my Spyder. Luckily I have a kit friendly MOT tester so I've scraped through for the last few years but it'd got to the point where hill starts are tricky and I had to leave it in gear when parking on even the slightest slope so time for a change. Reading up it seems GTM offered two choices, to go with Rover 220 calipers (which seemed to be their preference) or to use MGF. Mine was built with MGF but a bespoke version with bigger discs and the MGF calipers offset with one-off brackets. The problem with the MGF calipers is that the handbrake cables come in from the side. Not an issue on the MGF as the handbrake cables are routed via the subframe. On the Libra/Spyder the cables come down the trailing arm and then have to turn through a big angle to come back to the caliper. This curve causes friction and also causes the outer to split which then lets in moisture that corrodes the cable and causes still more friction. With the Rover 220 calipers the cables pull forwards from above and GTM put a lug on the tie rod mount to neatly mount the outer at the back of the top trailing arm. The mountings of each type are different. I thought it was just a case of obtaining the required bracket and a pair of the alternative calipers but it turns out its not that simple. To be fair its all in the build manual. To summarise, the Metro uprights have plain M10 holes at 82mm centres. The MGF calipers have M10 fine female threaded holes at 85mm centres. The Rover 220 calipers (which it seems are identical to MGZR and Honda Jazz so easier to source than R220) have M10 fine female threaded holes at 98mm centres. GTM solution for fitting the the R220 ones was to provide a bracket and spacers and to drill out the uprights and tap them to M14. The bracket is then bolted to the upright and the caliper bolted to the bracket via spacers the same thickness as the upright tabs (12.7mm). For the MGF calipers GTM drilled out the holes and provided eccentric bushes to bolt through and the caliper carriers still bolted direct to the tabs.
Anyway. I got a pair of MGZR calipers a while back and refurbed them with new seals and a coat of paint. I also got the required bracket and a pair of hub uprights with the M14 tapped holes (Which I didn't realise at the time I needed but glad I have them!) from Jamie as they were surplus to his needs after going Z-cars. To use Jamie's uprights I needed to fit new bearings and new ball joints too. Here are a few pics.
MGF caliper:

MGZR caliper:

Bracket for MGZR:

I've got the engine out at the moment and I dropped the petrol tank to change the hoses so that made replacing the handbrake cables easier. The R220/MGZR option uses shorter cables too, about 150mm shorter than the MGF ones.