Thanks for writing. There are lots of diseases that can affect crab apples. I'm really sorry, but I'm not sure which are most prevalent in the UK (or what the best treatments are for your area). I hate to say it, I can't really help much. I think it's best that you contact a local expert, such as an arborist, someone who owns an orchard, or the horticulture department at your nearest university.
---Justin, Senior Garden Editor, BHG.com