UPVC door operating problem

hi, first of all I have done the searches but can't find anything pertaining to this particular problem. If there are, then I apologise.

My UPVC door locks perfectly easily but wont unlock....after reading some of the other posts I opened the door and tried again, again the door mechanism locked perfectly but wouldn't unlock, however if I apply a little pressure to the small bolt that moves in and out with the key it will lock...(obviously only in the open position) it's as if it's popping out a midge too far.
Cheers for any and all help.

No suprise to me that there are no replies to my post, I think it only makes sense to me because I have a picture of the problem in my mind.

Can I come at it from a different angle....is the little bolt, part of the larger handle mechanism, or part of a seperate key locking mechanism, and can it be replaced seperately?
I am always wary of simply undoing lots of screws to have a look as I always dread the sound of something internal dropping when a screw is removed.
If I can ascertain this before unscrewing anything then I can look through the relevant posts for replacing the problem part.

There we go...this ones much clearer isn't it.

How about a picture on the web page, much more useful.