I've been using Stephen Groves https://www.stephen-groves.co.uk for a good few years now. Good guys, very approachable & helpful. Not that convenient to Derry but sure it gives you a chance to go for a good run. Without much of a detour, you could even take in a lap of the Ulster Grand Prix circuit on the way. BTW, remember MOT's in N. Ireland are carried out at government run MOT centres, not independent garages.