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A type of sawfly (Tenthredinidae). I Believe the galls are sometimes known as bean galls. Usually on White Willow and Crack Willow. One larva per gall which can also be green or yellow as well as red.
Thanks for this. I'm not 100% sure of the willow species but, as you can see from the leaf shape, white willow or crack willow are the most likely contenders.
For some reason this has only just appeared when I searched for galls. The revised ID is correct, it is caused by a sawfly not a cynipid.
Lat/Lng: 50.613310321681, -2.4594783782959
OS grid ref: SY675793
A brackish lake that used to be the estuary of the River Wey, now an RSPB reserve.