How can I fix my Hotpoint WD 52?

I have had this for some years and generally it works well. Had a couple of repairs over this time but otherwise reliable apaprt from one niggling problem. Over the last few months the door doesn't always lock when I close it thus preventing the chosen cycle from starting. Occasionally a firm 'shut' (or two or three firm 'shuts') would light up the locked door light but recently this has not worked. Flicking the little 'copper' plate just inside the catch aperture helped for a while but now the light just doesn't come on at all. Does this sound like a known fault or am I snookered? I can do a bit of DIY but if I have to remove parts to reassemble I could easily go awry. Usual dilemma - a call-out charge plus repair could well reach £100 and I could get an inexpensive new machine (not dryer as well) for under £200 if a sale is on.