it sounds like a lockstrip problem going by your discription of fault,try and take dood off at hinges,then order correct make,model,pz and backset of lockstrip or you will have a big job if you try and fit wrong one,hope this helps
shaun www.sdwupvcrepair.co.uk
January 2009
Automatic Answer
We may have found other answers which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix windows and doors.