I couldn't remember how to do this and nothing came up on a search so had to play around for ages this morning. (This phone keeps failing so messaging doesn't work unless you reset it).
So for anyone else wondering - To back up your contacts onto the SIM from the phone memory.
Go to Contacts - New Contact - Options (left key) - Advanced - Copy to USIM
Then when they are all copied you need to reset your phone.
Settings - General (tab) - Master Reset
When it restarts and you go through the basic configuration (date/time) it will offer to copy the numbers back off the SIM card.
Thought I'd blog this incase anyone else can't find this information.