remote entry not working

I have a 2001 UK ford focus, the remote stopped working so I thought, no problem spend £2 on a battery & it will fix it, but this hasn't worked. Is there something else that needs to be done like programming it? I only have the one key btw, & am worried Ford will charge me the earth to put it right.
Cheers for the help

Trie to reprogram the remote,it hapent to me a few times.
good luck!