you don't have to cut it down even if it is disease.

Apple trees need another tree nearby to exchange pollen or you do not get apples. Does the tree have flowers in the spring? If not then pollen is not the issue, it is just a strange tree.

Apple seeds have a wide range of genetic possibilities that cannot be guessed in advance. hat is why apple trees are grafted.

A tree that short after 20 years is a suspicious hybrid. Most standard apples (on own roots) I know of are 15'-30' high depending on environmental factors limiting growth.

A photo or so of the 'lumpy growths' on the trunk would be helpful

I bet the lumpy growths are root primordia- even without a photograph I figure it's about 50-50.

It is strange that a seedling is only 7' tall after that time if it hasn't been pruned or mutilated but it may be a genetic dwarf- or maybe it actually is diseased.

Whatever, the tree is probably virtually worthless unless it can be used for a grafting project. To me, only about 1 in 1,000 seedlings are worth eating, but not everyone is so picky.

Thank you very much all for your advise and information. The tree has never flowered and there are no there are no other apple trees nearby. I wondered if that was the problem but a friends tree has produced delicious apples for many years and is also "alone", so I began to doubt my theory on that. I do, however, need to reduce the size of the tree, although only small, it is on a raised garden (approx 3ft) and in a 5-6ft tapering gap between the back of our garage and our neighbours fence. I'll take a photo asap and post here.

Chezan...like others have suggested; Why not graft it over to something useful? You can (if you so choose) leave part of the original tree if it has some sentimental value. This year I grafted 3 varieties to a seedling I grew from a seed started from a store bought apple. All 7 grafts took and are growing like mad. If I can do it, anyone can.