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Thanks all.
Leg, I wouldn't have minded but this was 7:15 on the High Street in Oxford (which is really long), and there was hardly anyone around so it could have poped anywhere but aimed for me!
My mum thinks it was an environmentally friendly pigeon, that decided it would be better to drop on an unsuspecting victim than spreading disease and filth by doing it on the floor. My husband was laughing so much, mainly because I was still retching when relaying the story to him, that he had to hang up the phone and call me back. Even my nearest and dearest have no sympathy for me!