How do I change the lock on a house back door that is locked and the key won't work?

I have lost the only key to the back door and now need to replace the lock. It's just a standard door lock - the kind where you insert the key in the door handle and the latch moves out the frame as you twist the key. I took the handle off and all i can see is the hole in the wood where the key goes into the lock. Obviously if I could open the door it would be simple to remove the lock and replace it but I can't. Is there anyway of getting round calling a locksmith to at least initially opening the door (I'm sure I can replace the lock myself). My kids are desperate to go out and play on the secure back garden and we don't really have the spare cash to get professionals out. Any suggestions I would appreciate enormously. Thanks.