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
21:53 Sun 29th Mar 2015
A good alternative site is found by typing in verbatim the entire clue given and click on the website "DANWORD" Not only will it give you the answer but scroll down and you will see many more answers to the same crossword.