>>>Not sure of her surname
Does that mean that you can't remember her surname (as it was when you knew her) or simply that you recognise that it might have now changed through marriage? If you only knew her as, say Susan, you'll probably have great difficulty in tracing her but if she was known to you as, say, Susan Macheson, you might stand a reasonable chance of finding her.
A useful trick in locating people can be to forget about searching for the person directly and to concentrate instead on looking for someone who will know where she is. For example if you remember that Susan Macheson's brother was called Zebediah, then looking for Zebediah Macheson could be a relatively simple task.
192.com lists everyone in the country who is eligible to vote and who hasn't marked the box (on the electoral registration form) to have their name excluded from the edited version of the electoral roll. It also lits everyone whose name appears in the Phone Book. Some information is free but most is charged for. (However you can still see, before you pay, whether there's anyone with the relevant name on their lists):
http://www.192.com/
Chris