I took it to an amp tech and it got transformed in two stages (one would have been cheaper!).

First stage was getting it working as it was. New input transformer, re-cap and new valves. New original transformers are unobtainable, so it was either trying to find a used original at the same cost as the amp, or a custom wound transformer at a similar price or adapting a different transformer. My tech went down the last route, fitting a Marshall transformer and deriving one of the voltage supplies (the original had its own winding for this) via a resistor circuit, as Marshall do. He also replaced some dodgy pots and fitted an IEC socket to replace the captive power lead.

So I took home a working, but very noisy amp. They were known to be noisy amps, mainly due to the active tone controls.

He said that there was a circuit mod on the web, done by the original amp designer, that changed the EQ to passive, along with a few other tweaks that turned it into a very similar amp to a Hiwatt (as the designer had worked for Hiwatt). After living with it for a week, I decided the hiss was too much (and the active EQ a bit weird sounding), so it went back for more mods.

He spent a lot of time trying to make it as quiet as possible. He would have liked to have fitted a choke, but there wasn't room on the chassis. It is a very compact head, probably 2/3 the length of a full-sized Marshall, so not a lot of room inside. The awful hiss has gone, but there's a fair bit of hum still. But that gets lost when you play. It is now a very loud, great sounding amp with that Hiwatt character, making it very easy to get those Dave Gilmour clean tones. Used with a speaker attenuator, or just turned up very loud it makes a great drive sound, very different from a more crunchy Marshall sound. It does have a lot of bass, and I have to keep the bass control at minimum (I rotated the knob so that minimum was at 12 o'clock so it doesn't look strange).

I now know a lot more about valve amp circuits than I did, so I plan to see if I can make it quieter still once I've finished reading a couple of books on the subject. Also see what type of taper the volume pots have, as it goes from quiet to very loud with a few degrees of rotation, and maybe a high pass filter to cut out excessive bass and reduce the 50Hz noise.

The two-stage approach to the repair made it an expensive project, more than it would sell for, but I do now have a working, quite rare, 70s amp.