Cracking photo Iain
I think richard is spot on, it looks a little bit too light for an MB, could just be immature though (the eyes also suggest this), as it is too small for either MB or OU. The dot is there on both flies.
At a guess i'd go with Olive upright though, i'm sure the MB's tails are a different colour
(speckled possibly)
Get some more pics though, i always intended to follow up the Videos with a series on the natural flies of the river and try to make a decent online library of bugs for people to peruse, but so far fate has not been kind
Cool
Sandy