There may be several reasons, the trees are quite young so they need a lot of water, London Clay soil will not help, did you, when planting dig a large enough hole and fill with a good compost before planting, if not you can scrape away some soil and dig in some good soil, compost or a good feed. A very good gardening book 'Be your own Expert' which has verious books under this title, fruit, shrubs, houseplants etc. with a section with pictures on whatever is attacking the leaves, excellent book and not expensive. Each year in winter spray both trees with a tar wash, from any garden centre, this will protect the trees from next years infestation or creepie crawlies!!! for the time being try to give a good water 'used' water is ok in the first years trees need a good start. Dont despair we have had a very hot time recently, which is made worse in a city.