No need to feel guilty, Sue - and it's most unlikely to ever happen again, sort of lightning never strikes in the same place. There's really only a limited amount one CAN do for birds, they are so different from us, their bodies are so small, they stress so easily, and unless they get one of the easily-diagnosed and run-of-the-mill things for which proven remedies have been developed, often the kindest decision is to PTS, as you did. Hens don't have a very long lifespan in any case, do they, compared with other creatures - I know this hen was quite young, but you gave her a good life and what she had was a reasonable proportion of what she might have achieved if she hadn't become ill. I think sensible economics has to come into it as well at some point, though this will vary from person to person, as does the health of the whole flock if there is any risk in keeping a sick bird alive. Death isn't always the worst thing - I hope someone will knock me on the head before I go totally gaga and incapable, rather than keep me in a state where my life isn't worth living any more.
I agree about your frustration at paying for an inconclusive PM - isn't there a lab. near Oxford where dead birds can be sent for forensic analysis? I thought this was where yours was going, didn't realise the vet having a go herself.