Lime Nail Galls.

'Tis the season for galls.  These were on a lime tree in the avenue behind the cottage.  RHS:

"Lime nail gall mites are microscopic animals (less than 0.2mm long) that feed on the foliage of lime trees (Tilia sp). The mites feed by sucking sap. They secrete chemicals into the plant tissue that causes the leaves to produce the galls. The mites then suck sap from plant cells lining the gall structure."  Apparently they don't cause any noticeble harm to the trees.