Thanks for the all the valuable advice!
I've just signed up for an account with sipgate and I now have an 0845 number for my business :)
I'm thinking of buying the Grandstream BT-101 from them for £45 (http://www.sipgate.co.uk/voipshop/grandstream/grandstream_bt-101) but until the expenses claim goes through I'll be playing with softphones.
Wish me luck configuring my router...
Cheers
Ben