Old apple tree care: hollow repair, pruning, wound sealing

We have an apple tree in the yard that my parents planted, so it is around forty-five years old. It still sets a solid crop of late keeping apples every autumn, and I have no intention of taking it down — I want it treated properly. The trouble is in the trunk. In the main fork, about four feet up, there is a hollow. The opening is oval, roughly six by eight inches, and the cavity runs back about two hand widths. Inside it is wet, packed with soft brown rot and crumbling fibre. After rain the water sits in there for days, and in spring you can smell the sour wood from a few steps away. I am looking for someone who actually works on trees, not a handyman who will smear the hole shut and drive off.

Here is the work as I see it. First the cavity gets cleaned back to sound wood with a chisel and a scraper, all the punky material removed until you reach firm, pale timber. Then we decide together: leave it open with a drain at the bottom so water runs out and the cavity can breathe, or close it with a flexible filling over mesh. I am not asking for concrete the way it was done years ago. A rigid plug does not move with the trunk, a gap opens along the edge, water runs in behind it, and the rot keeps going where nobody can see it. The wound edges should be trimmed cleanly along the living tissue, and the cuts treated with whatever you normally use — tell me in advance what that is, because the fruit is eaten by children.

The second half of the job is sanitary pruning. There is deadwood in the top third, two limbs rubbing each other over the path, a few watersprouts growing into the centre, and one old stub from a bad cut years ago that is rotting as well. The crown needs thinning, but no heavy reduction: I am not willing to lose a quarter of it in one season, the tree is old and will answer with a forest of new shoots. On timing I am aiming at early spring before the buds swell, or late autumn after leaf fall; I do not want this done in summer heat, when wounds dry badly and pests are flying. The high work must be done with the limbs roped and rigged down — there is a greenhouse and the neighbour's fence underneath, and nothing can be dropped there. In your reply please tell me your experience with cavities, how you clean them out, what you suggest instead of concrete, and how long the whole job will take.