Thanks John. I put "sawfly on Alder" into Google and found the following:

Larvae of Platycampus luridiventris feed on the underside of leafs of the three alder species Alnus glutinosa, Alnus incana, and Alnus viridis.The body is dorso-ventrally flattend and thus, these cryptic larvae are hard to detect. P. luridiventris has an larval development time of up to three month and can be found till late autumn.

The picture looks quite like mine too so that may be the culprit. Isn't this techno-sharing wonderful? :-)