It's Pear Rust, new to the UK in the last couple of years. This particular form needs a Juniper plant to complete its life cycle, so if you have any Juniper species in the garden, its worth considering removing them. There is currently no effective treatment for this fungal infection, unless you happen to already possess some Westland Plant Fungus Control concentrate - this product was withdrawn from sale in June this year. Otherwise, remove affected leaves promptly to prevent spread of spores, bagging them up straight off the tree and disposing of them away from your garden compost. Bayer Garden Systhane Fungus fighter concentrate MIGHT have some effect, but its really intended as a treatment for pear scab (source info on fungicides RHS)