What's wrong with my cherry tree?

Looks like a nutritional problem to me, as I can't see any aphids or signs of fungal/bacterial disease symptons. However, many of the leaves look pretty healthy, so it may not be that serious.

Leaves naturally turn yellow when they are senescing, with the breakdown of chlorophyll and the recycling of nitrogen containing compounds. Developing leaves also turn yellowish if they haven't got enough Nitrogen to start with. When a plant becomes generally short of nitrogen, it starts to withdraw it from the oldest leaves in order to try to keep up a supply to the newly growing/developing leaves. So the oldest leaves should go yellow first. That this is happening isn't 100% clear from your photos, so I could be totally wrong . Also, it could be another nutrient (e.g. Magnesium) in short supply. What's more yellowing can be caused by various other things like waterlogging.

Anyway, maybe a general liquid feed (N:P:K + trace elements) would be a reasonable measure to take. If the problem's nutritional it should stop any new leaves going yellow. The ones that are already yellowing are unlikely to green up though.

Thanks. There are plenty of drainage holes in the pot but will get someone to help me tip it to make sure they aren't blocked. Planted it earlier this year in mpc and mixed in a few chicken pellets.

I wasn't too worried when a few leaves went yellow but with so many I was worried it might have a disease in unaware about. Will give it a feed and see what happens.