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I've had some email feedback from Limoniid expert Dmitry Gavryushin from Moscow who says: I could only confirm that the fly looks more like viridipennis, please check my field observations of both G. viridipennis and G. acheron on the same fallen aspen tree in June, 2010: https://diptera.info/forum/viewthread.php?forum_id=5&thread_id=30791

Thanks to this, I am even more sure that my fly is Gnophomyia viridipennis as the halteres are more yellow than in the G.acheron images Dmitry posted and G.acheron is also not recorded from the UK.

We have scarce records of G. viridipennis here (with G. lugubris sometimes given as a synonym) and even fewer of G. elsneri (which looks quite different) from just one confirmed location near London. G. acheron seems to be an Eastern European species, so unless it has been missed in the UK, is unlikely to be a possible ID, and I think G. viridipennis remains a very likely ID for my fly, though further corroboration or comment from experts would of course be welcome.