I would reverse the previous claim and pay the tax based on the profit I'd made. I don't think I'm smart enough to evade the tax man. Plus I believe in a social system where you give what you should and take what you need. Evading HMRC doesn't sit well with me. However, I also appreciate that not everyone shares my thoughts on this. But then, thats up to them.
Things get rather muddy if I brought said bass new in 1965 (lets say for equivalent £1k). Claimed WDA as it depreciated. Retired in 2013 and sold bass for £5k as I no longer needed it. I see how that would work in principle, but is there some kind of time limit placed on this or is it a grey area yet to be tested (I appreciate this wouldn't work for other trades)?