Although their website - http://www.brlines.com/index.html doesn't say anything about Minitrix repairs, their list of other products does show some Minitrix/Trix items so hopefully they may be able to do them? I would've suggested BR Lines as the most likely to be able to repair them though! (Not that I have had any problems with my Minitrix loco's but have had a Dapol loco repaired by them, and found they provide a good service!)
I spoke to Bob at BR Lines today. They don't do Minitrix repairs any more, but he did put me onto Tom at Model Railway Doctor as mentioned elsewhere on this thread who is able to help me. They cannot supply just spares though, as they really are in short supply now, but will undertake repairs inc replacement parts.
I'm happy I found this thread as I've managed to shear off a con rod/driving wheel hex bolt on a Britannia I've bought while stripping down to service her. It was all going great until that happened, she's an early example that did limp & jerk along without traction tyres, all gunked up inside, but the cleaned parts (after IPA bath) seem in good nick.