I had promised to post back on the expansion vessel from Post #26. The recent posts on this thread reminded me. I'm throwing all this down here in case it's useful to someone else who finds themselves in a similar situation;
Anyway taking on the advice I received earlier on in this thread, I ended up getting quotes from multiple local engineers to repair the expansion vessel that i mentioned in post #26, with options for them ordering the parts or me ordering the parts myself. In the end I got a local engineer and ordered new, original-manufacturer replacement parts myself;
What I paid was;
> Expansion Vessel £48 vs £126 quoted by originally contacted "UnNamed London Based Keston Specialist" from post #26
> Relief Valve Kit £34. vs £55 quoted by originally contacted "UnNamed London Based Keston Specialist" from post #26
(all costs ex. vat. all 'like-for-like')
It took Chris, the local engineer I went with, about an hour to fit it, have a chat about doing some more work in the new year and clean up after himself.
In the end I saved £80 on parts, some on labour and found a good local plumber (who doesn't charge callout). I'll be getting him to do all my heating work in the future.
As other posters have pointed out, I wear the risk that the parts might be duff, but - touch wood - everything's run smoothly since the work was done.
Good for you. Nice to hear you have found a good local engineer that you can trust.