Yellow spots with "warts" in centre on pear tree leaves - what are they?

[TuttiFrutti]

Our lovely old pear tree has suddenly developed horrible yellow marks in the centre of every leaf, with a lumpy thing like a wart in the middle underneath. Does anyone know what this is? Some sort of blight?

sounds like some kind of beastie - if you cut into a wart is there a caterpiller or grub in it? (now dead of course!)

Ooh how horrible - I hadn't thought of that. Will go and check.