I'm just wondering in what way the growth is 'mysterious'.... it must have been in flower during the last few weeks, pity you didn't post a pic then, but yea, it might well be a crabapple.

Did you mean this is the first time it has flowered? It is a big specimen, at least six foot high, if the foliage we see mostly belongs to a tree in a neighbouring garden. If a neighbour has planted a tree that is in the same pollination group as this one, that could explain its sudden successful fruiting. There are lots of fruitlets there and I would not be surprised if a lot of them fall off in the 'June drop'. It is hard to say what it is without seeing the blossom so please do keep us up with what it becomes in the next few months. Apple days are held all over the country and the growers there are usually able to give a definitive answer if you take a sample of the fruit along. Google apple days to find one near you around August time. If you load an answer in the comments box we will be able to relate the answer to this question.

Thank you both for taking the time to reply to our query.