Thks for the great advice. Good to know there's a few ways around this.
Did some measurements today and it's clear what's happening. Attached photo is my markup of a hybrid advertised as 307-0386-00 - has same pin setup as mine but may not be an -00. Resistance measurements done in circuit but with focus pot leads disconnected. The hybrid is definitely cracked right through - separating the +50V and ground rails from the -3kV supply. With scope on, the Cathode Regulator test point is reading 118V (should be 95+/-5) and the input to pin3 on U1624 is sitting at -5.4V. Cathode Reg pot has no effect on the 118V reading. So I surmise what's happening is that, absent the ground and 50V rails the -3kV supply feed into U1624 is being clamped at -5V by CR1626 (just as well!), and there's no regulation of the 3kV happening via U1624. That would explain the fuzzy and variable focus.
I was pondering last night whether, absent a replacement, it might be possible to glue and bridge broken trace on the cracked hybrid as suggested by James. However, the good news just in this minute is that Sphere has in fact got one in stock! Just have to pay the delivery cost from USA, which is always high to Australia. I prefer replacing like for like when I can. Nice to know my re-cap work won't be wasted - I was rather pleased with that. I do like the build quality on this old machine and it's nice to keep it looking good as long as it doesn't break the bank.
I'd like to have a go at a glue and bridge repair of the old one, by way of a trial and - if it did work - to keep for a rainy day. Would appreciate a few more details on how precisely to best try that and any pitfalls/problems I might encounter. Are you sure a fix like that would survive the HV load? I don't have the expertise to judge that.