It should be possible to source a replacement, compatible, brake master cylinder from a reputable manufacturer, like Brembo or AP Racing and have it fitted by a competent specialist.
OK, it will cost a lot more than the FOC replacement that morgan will eventually provide, but what price the frustration of no car this summer...? It might even improve the pedal feel, which from my experience is OK, but not special.
I know the frustration, Tarka is sidelined and no one at MMC seems to be interested, probably because they have more pressing issues.
Peter, if I had a Morgan, I think this is the route I would go down?
Summit Racing sell new Master Brake servo's with adjustable pressure, for £300 (so the brake feel might be better than what some folk have reported?)
I do think that a proven unit would give peace of mind and what if this manufacturing defect pops up again, as per the radiator leaks?
I do feel sorry for CX owning Morgan owners though...it is a real shame and long term will 'hang about in motor folk law...like a bad smell?'