Thanks Paul ,on the back of your ID I googled a while and it appears to be a Dioctria sp ,still can't pinpoint which one though.
I saw it at Stockers lake Rickmansworth just outside London .
Just to add to this discussion. Here in Northants it has cropped up several times in the last few years in ancient woodlands - mainly oak/ash. The records have been forwarded to the Larger Brachycera Recording Scheme but I don't think that dataset has been loaded onto the NBN Gateway. The Centre for Ecology and Hydrology published a provisional atlas for the larger brachycera in 1991 and may still be available.