Having seen the picture of the underside of the leaf, I still think it's apple leaf scab.

Thanks for the help. I've looked on the RHS website for treatment suggestions.
The tree is very little, not 3', we only bought it and four other patio fruit trees about three moths ago.
I can't prune out the branches with the affected leaves, because it doesn't have any branches yet, just the central trunk.
Hopefully the leaves will fall off and we can burn them.

Burning the leaves is the most effective way to control it, but you must make sure that the spore don't get back to start the process all over again. If that fails, then you could try a copper Bordeaux mixtures, which is recognized as an organic control and used as the name implies, on grape vines, or a chemical fungicides as a last resort, but I'm always wary of using them on things you intend to eat at they need to be applied when the tree is in leaf.

Having said that, your tree obviously isn't producing fruit, so this could be an option.