@ Gus,
This is a suggestion, not a recommendation, I've never used this shop, but Harry Hall Cycles (
67-69 Whitworth St W, Manchester M1 5WQ) have good Google reviews and claim to have been dealing with Bromptons for 35 years. You could give them a ring (0161 236 5699) and see if they're willing to help. I think a dealer with folding bike experience would be better than one with none even if they don't specialise in Mezzo's.