When I lived in Little Kingshill I let these guys service my 1967 MG Midget .
http://greatmissendengarage.co.uk/Miller Commercial Motors in High Wycombe did the Mog, but Brian Miller (a Mog owner) has retired though I understood the new owners might still be doing Mogs.
I haven't lived in Bucks for 3 years now so caveat emptor on my recommendations.