Thank you everyone for your advice! I have now replaced the lock and all is working fine!
Should anyone else suffer with the same issue, you need to remove the rear seat to get to the screws at the bottom of the door then you can remove the rest of the screws and quarter cover. The panel can then be removed to acces the inner panel, you have to remove the electric window motor and be careful of the plastic housing holding the cables! (I broke mine but was able to glue it back on).
There are 2 philips screws on the back of the white plastic housing, you then unclip the housing and access the lock! Then refit, this took about 2 hours not bad and saves alot doing it yourself, I got the lock on-line for £62.40 including p&p.
BEWARE! There are 2 screws on the outside of the lock, 1 is the white screw (this is the child lock). The other screw is black plastic and controls the lock, this is what had snapped so lock wouldn't open!!!!!! Don't let anyone else set your child lock or someone might turn the wrong screw and mess up your lock!
Thank you again.