I had rather hoped that I would be able to key out a Willow catkin, and there may be enough info all round to identify the tree with bark and brach characteristics, but Little Stace had this to say: "In many cases catkins and both young and mature leaves are desirable for identification; keys based on catkins alone are not satisfactory. Mature leaves should be collected in Jul-Sep"
Jul-Sep!!! OK. I'll maybe know what sort it is come the autumn, as long as I can remember which tree it was. Here's a pretty picture of some catkins - Salix sp.!