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The larva looks very like the Ichneumon wasp genus Polysphincta, which are more often seen on the backs of Tetragnatha spiders. There's a review of the group by Gauld & Dubois (2006) which is downloadable from Google: highly technical, but the later part summarises the distribution and known hosts of all the different genera close to Polysphincta.
Thanks for that, I shall have a read of the paper :)
Lat/Lng: 51.8, 1.0
OS grid ref: TM1118