Destroyed details of bank and building society accounts

[Germs]

Sadly my mother has gone from endearingly ga-ga to being away with the fairies. I have recently obtained power of attorney over her estate and need to liquidise assets for the looming nursing home fees. As far as I can make out she decided if she destroyed all details of her savings/investments etc., the tax man wouldn't get his hands on her money! I've no real idea where to start, her neighbour (only semi ga-ga) thinks she recalls talk about an account in Sark, she also thinks she remembers mention of account(s) in Australia. I suspect her assets will be a mixture of banks, building societies and shares. Am I correct in thinking there are centralised points for the UK where I can make enquiries, but what about elsewhere? Any advice gratefully received.