One of the keys of my piano has suddenly started sticking. Do I hope it will rectify itself or should I call in the experts?
It is a good make and I do have it regularly tuned.

Katie

Occaisonally one of my piano's keys will stick and I just open the lid and gently touch the hammer. It doesn't seem to cause any harm and always sorts out the sticky key.

Can happen by various causes. I'm often called to go to clients for sticking keys.
Very often, it's the key itself that sticks, because the felt in the hole in the underside of the key has swollen and can't slide over the metal pin as smoothly as it should. In this case, the key feels a bit stif and stays down or rises only slow. Pushing the key a couple of times to the left and right side can help make it a little more loose, but your pianotechnician will know a better way to fix the problem
Other possible causes (to be treated by a technician) :
-(especially in old grand piano's) the plumb pieces in the keys are sticking one to another.
- oner or several keys in the middle register are sticking to the bar in front of the keys.
- some small element in the action can be stiff or even broken, one of the springs has lost its power,...

I hope I'm not doing any damage by tapping the hammer...

Depends on how hard and how often you do that, I guess
But pianos are supposed to be very resistent.