That's because Martin Hood, who owned that domain name, has failed to re-register it (by paying the relevant fee) as he was required to do by 25 February if he wanted to keep using it:
http://www.nominet.org.uk/whois/lookup?query=trivkey.co.uk
Thanks for the info Buenchico it is a pity as I always used this site to cross check my answers,maybe he will hopefully decide to do another one in the future!