- Should I have descended 'blind', thus ignoring quadrantial rules ?
Quadrantal Rules would not apply as you would not be maintaining constant Track and constant Level - However I would not want to descend through a cloud layer (as you describe) blind either.
- Was I right to ignore the 'standby' instruction (albeit after four minutes) ?
Yes, I don't think I would have waited so long
- Is it reasonable to expect a new ATC unit to see the problem that I was going to be faced with ?
Probably not if you haven't told them - you don't know what, if any information, the previous ATC had passed to them.
- I don't think I had any service agreement whatsoever with the new ATC unit at the point of 'standby' - as we had not agreed anything at that point, but is that correct ?
I would agree - correct
- Anything else I could/should have done ?
I think you have illustrated one of the problems of IFR in Class G airspace, particularly as you were also IMC.
Perhaps you shouldn't have had a Plan A which took you through Controlled Airspace without a more useable Plan B.
What was your initial call to this station?
Even if not technically correct I would suggest something like:
"G-ABCD, On Handover from ZZZZZZZ, Squawking 1234, FL55, IMC, approaching your Zone, Request Zone Transit"