There is an offence of 'stealing by finding'.

I found (!) a camper van that took my fancy, quite clearly abandoned outside a scrapyard to be towed in on monday morning when they opened.

I spoke to a police friend, she told me about stealing by finding, you need to make sure that have made reasonable efforts to trace the owner and can demonstrate the fact, this would become your defence if someone tried to claim it.

I'd reccomend posting an advert in the local paper and maybe the Times, keep a copy, take photos of the notice you posted to the mast, record letters you sent to the HM, register it in your name!

Good luck.