A woodpecker?

Can you expand your question a little please. Is it a small hole perfectly round or does it show signs of rotting around it. How large is it?

Not sure if it could be woodworm, as there is sawdust coming out of a small hole near the base, some of the bark has rotted away. also the tree is quite old.

It could be a bark beetle, one of the Longhorn types the larvae of which burrow into wood. The sawdust is significant though as to the exact agent causing it?

Apple trees a reckoned to last around 50 years before falling to pieces and rotting. Do you think it could be this old give or take 10 years?

Yes it could be about 50yrs old, was here when we moved to this house 31 yrs ago. It has not had very good yields over the last few years, so i guess its on its way out, unlesss there is anything i can do to save it.
THANKS .

Save from pruning very hard back into heavy wood next spring to stimulate new growth, this of course being dependent on the condition of that trunk. This is a practice used in the past to gain another ten years or so of productivity out of an old tree.