Many thanks to bd139 for posting the tip-off about the HP 6209B power supply. It arrived in Sweden today via DPD for just £15 shipping, which is probably the cheapest shipping I've ever seen from the UK to Sweden for a 6+ kilo package.
The supply works just fine! It reads a little under on the built in meter, but it goes all the way up to 339V without any trouble. It has a few dents and some scrapes to the paintwork on the front panel but is in fairly good condition otherwise.
Before I powered it up, I opened up the case and blasted out all the dust. I was also glad I had downloaded the manual as the strapping on the back of the unit was set for external constant current programming. The inside, after blowing out the dust looks pristine.