I was able to deduce that they were the caterpillar of a geometrid moth by the size and shape, but got frustrated in getting an identification. Caterpillar identification is difficult, especially the geometrids. There are few good sources of information, and one of the best - the UKleps website - involves trawling through hundreds of photos on each species.
I posted the pictures onto the UK caterpillars group on Facebook and quickly got some suggestions. Swallowtail Moth was one, which looked promising but not quite right. Another was Scalloped Hazel, but this is a species I had already considered and ruled out. It was not helped by the quality of the photograph (the white background causing my camera some difficulty), but Shropshire moth recorder and caterpillar genius Tony Jacques came up with the goods. He told me to look at Light Emerald (Campaea margaritaria), and he was of course absolutely spot on.
|Light Emerald (Campaea margaritaria) larva|
Light Emerald is a species I have recorded several times in the summer, and this is one of the species that overwinters as a caterpillar, so the identification was not a surprise. It is a species I expected to find this year in my 1km square, though perhaps not for several months yet. This is the first one I recorded in my garden, back in 2012.
|Light Emerald (Campaea margaritaria)|