Bugger!
Pretty bad luck really - *most* of the time when a company goes bust the mail just gets returned and the domains get dropped and can be registered by anybody.
In some cases (yours!) the domains get transferred to the Office of the Treasury Solicitor and get auctioned on
www.auctionurls.co.uk for a MINIMUM bid of £500 and a £250 transfer fee! (edit: it's not listed at the moment)
As mentioned you can do the legal stuff to prove it was supposed to be registered to you but
I'm a Nominet member but any of the reputable guys (www.123reg.co.uk,
www.gradwell.com,
www.totalregistrations.com) can accept a domain for about £10. The £80+VAT is if you want the domain to stay on Nominet's tag, which is obviously an expensive option for them.
Nominet are an excellent registry but they definately do everything by the book with no exceptions.
Gareth