Ford Ka (1997) door mech probs - how do i fix?

I found a while ago that when unlocking my door it started to feel stiff and offered a bit of resistance. It slowly got worse and now completely wont budge when I attempt to unlock. Locking it absolutly fine. Is it an easy thing to fix or should i take it to a garage to get repaired? I dont want to take off the panel and end up making a mess of it and doing more harm than good. Has anyone any ideas as to what is likely to be causing the trouble?
Many thanks in advance.

Mike
January 2005