Crab Apple Tree Scab!
It does sound like your tree has got a fungal disease possibly Apple Scab but I’m slightly puzzled by the fact that there is no evidence of disease on the leaves or the tree. The most serious evidence of scab is reduced vigour caused by early leaf fall (which you have) and by scabby marks on the fruit & leaves. You said that your tree has not produced fruit so you will only be able to tell by the leaf drop and the leaves. A few crab apples never produce fruits so it may be that you have one of these varieties.
Apple scab infections occur between late April and early June, you can spray with a fungicide during this period however once a tree is infected it is too late to use a fungicide to control the disease. Harrods do sell an organic product called Bordeaux Mixture which is ideal to spray on smaller trees but it would be almost impossible to spray your 12m tree without spraying yourself and other trees nearby. Unless you used the services of a professional to spray your large, old tree which could be expensive but it maybe worthwhile calling a tree professional for advice.
I am going to suggest good hygiene over winter and wait to see what happens with your tree in the spring.
The fungus spends the winter on fallen leaves and any infected shoots if these are not pruned out. The disease is spread by airborne spores and splashes from rain drops which spreads infection throughout the growing season. So start with raking up and burning any fallen leaves and removing an diseased foliage/branches if possible.
You can apply a Fruit Tree Grease now as a preventative measure in case any pests are present. Tree trunks are a key route for many pests to attack fruit trees, that's where the Fruit Tree Grease can provide an easy way of monitoring pest activity. The grease is brushed on in 10cm wide bands around 18cm above ground level and is applied in the Autumn and can be renewed in February. The grease needs to be well worked into any cracks or crevices on the tree trunk for maximum effectiveness, providing a simple way of monitoring activity of any unwanted pests.
I would then give the tree a good mulch feed of organic matter in the spring, if your tree doesn’t bounce back it may be that you have to start again with a variety that has more resistance.