The tree will only retain the fruit it thinks it can carry. Your tree thinks it cannot keep any, so it is either too young, hungry or thirsty. Obvious answer to that.

What variety is it? Is it self fertile? Does it have a nearby pollinator? Is it in a pot or the open ground?

Other than that, as above reply

I think it’s a James Grieve. It used to have a Cox nearby but that wasn’t happy either. I wonder if it’s the stony ground as the houses here were built on a brickworks. It doesn’t look very happy these days, that’s for sure. We usually treat the soil in the springtime but neither of us has had good health so it didn’t get done until a month or so ago when we employed a gardener.
Thank you

This usually happens in June and is known as June drop. The tree has 'hormones' that sort out how many fruit it can carry. Too many developing fruit and as Owd says lack of water/food etc.