Just an update on mine.
It certainly is MDA.
PSU really wants to be driving a full PC (rated 66W) - anyone who grabs one should either do this, or plan to hack/install a single +12V DC line for the monitor and disable the full PSU inside.
V-Hold - adjustment helps, but I can't lock on, and I'm getting a strong pin-cushion effect. The lack of V-Hold varies at random.
Mine will likely go back in it's box while I finish my HMM-1200 MDA repair, and then become a project for education rather than requirement.
Full circuit diagrams + test points and readings/adjustment instructions are available on the net (yey Amstrad)
If anyone else grabs some of these, let me know how you get on