I guess nobody else can identify this either. Is it possible the caterpillar was diseased and consequently the butterfly did not develop its normal colouring? And if that is possible could it be that the butterfly is smaller than normal for the species? If a disease could disrupt the metamorphosis in such drastic ways then I suppose it would be impossible to identify the species.
I agree there are lots of differences. Here's a small heath I photographed in Spain this year.
The body is very hairy while the mystery butterfly's body is relatively bald. The eye is prominent but there is no similar sized eye in the mystery butterfly. The hind wing shape is quite different. The mystery butterfly has a sub-marginal line on the forewing outer margin that the small heath lacks. The small heath has one black spot while the mystery butterfly seems to have the remains of three spots. What would be good would be if one of the museum experts could tell us if these differences could be explained by age or not. I obviously have no idea.
(There are also similarities. It is not obvious from this photo but the front edge of the forewing has a section that is straight, almost concave. The legs look similar. The antennae look about the same length.)
What a bunch of dummies we are!
I have shown this to my lepidoptera colleague this morning who points out that it is of course a moth!
I have to admit that my eyes were drawn to the wings and completely missed the antennae.
My colleague, Martin, goes further and identifies it as the day-flying geometrid moth Isturgia famula (Esper 1787). Looking at the following image you would have to agree: http://pathpiva.wifeo.com/isturgia-famula.php
Thanks. I do have to agree with the identification and (in my defence) I did express my unease at labelling this as a butterfly in my original question. I've just read Chinery (Insects of Britain and Western Europe) on butterfly and moths and he states that there is no single feature that separates butterflies from moths which is "an artificial split based on simple observations and having no real scientific basis." So maybe we aren't such dummies after all.