I took this beside the River Stour in Dorset a couple of weeks ago. I think it may be a longlegged fly but I am not sure. Would be great if anyone knows for certain what this was? It was very big for a fly and looked quite mean
That's a robber-fly, one of the Asilidae. There are about 30 British species - an expert should be able to get closer. I'm not an expert but I recognised it because one turned up in my sweep net last week.
Well I think so - asilids have short beak-like mouthparts, whereas empids have a long snipe-like mouthparts as in this image. My money is still on an empid. Empis tessalata would not be far off the mark.