It's a gall - Lime Nail Gall Mite to be precise. Doesn't harm the plant, just looks weird... the mites get on the leaves and these are the result of their activities.

thanks for the fast response

Goodness!

Well I never! What plant has the ' mite' chosen to inhabit?

Looks weird.

You guys must not have spent time roaming the parks or country side as kids - I well remember noticing this stuff on tree leaves (usually low down suckers off trees, given I was little) and thinking how peculiar it was, must have been about 8 or 9 years old I guess.

I grew up in very rural North Norfolk and know lots about the country side and rivers but I never saw this that I remember.

Looks like a Linden tree (aka Lime tree) Pam.
I've got to admit it's the weirdest gall I have ever seen!

Most commonly occurs on Lime trees - hence its name, but you do find it sometimes on oak and other trees. It is a funny looking thing, which is probably why I remember it from childhood.