This chap has been a very occasional visitor in my garden but I've never managed a good shot until this morning. Must be my lucky day.
I think it's a Papilio eurymedon, commonly called Palid Swallowtail.
PS Now having doubts because the above is a US species not normally seen in Europe. However Papilio Machaon, the European cousin, is much more yellow and has twin red "eyes" on the inner edge of the lower wings.