As to the issue, Real Electronics in Sheffield are the Mark Bass appointed repair agents. However, they don’t do component level repairs - if there is a fault they’ll swap out the board, which makes it expensive if out of warranty.
I had a fairly unsatisfactorily experience with them - I had a intermittent fault with my CMD121P, sent it to them, they said there was nothing wrong, replaced all the pots, charged me £125, and then the fault came right back again.
I ended up buying a second hand Little Mark 3 and replacing the head with that.