If the 'character' really wants to buy your caravan then he maybe won't mind paying you with a bank cheque for example. Its as good as cash and clears straight away.
You could also get your self a counterfiet detection pen which comes out one colour on genuine cash and another on fake.
you could also get yourself a uv led keyring and you will see an ultra violet feature on the notes. Worth spendin �5 on.
Check this - its the bank of england website which shows you security features
http://www.bankofengland.co.uk/banknotes/secur ity/security_20.htm
http://www.bankofengland.co.uk/banknotes/secur ity/new_security_20.htm
P.s i would rather look and idiot in front of someone i didn't know and would maybe never see again than be �3000 out of pocket.
Also look on this website. Crime provention products.
http://www.c-p-p.co.uk/default.asp
Hope this helps