Sorry, but the question is too hard. I think you need a grown up.
I asked Tasha (who was a tax accountant once) and she said something but it has all gone a bit fuzzy.
Thanks! Claire W has emailed me - she's also got some tax exams / experience. Her answer fuzzed my brain too!
Given that I could save £thousands in (legal) tax avoidance (... meaning I'll never get knighted? ;) ), I may spend a few £100 on an appointment with a pro tax expert soon, to see what's best to do ....
See if he is serious when you come up with a fair price.
Setting a fair price will be hard.
See a tax guy.
Buy a new golden egg laying goose.