small, yellowy mainly blackish
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I'm not sure if it's a horntail - the photos I've seen show the male as mainly yellow. Try looking at the following website and see if what you saw matches: http://www.bioimages.org.uk/html/p6/p69085.php
You probably have some sort of parasitic wasp. Taking a bit of a guess at the colours and size from your photo (and your location), I'd take a wild stab at the Black Slip Wasp (Pimpla instigator) - it's mainly black with redish-brown legs, around 2cm long and fairly common in the UK. Search google or Flickr for photos. There are lots of similar insects though, so I could be miles out!
If you can say which bits were yellow, it might help with the ID. If you get another chance to take a photo, try it with the flash to see if it shows up the colours better.
It's not Urocerus gigas (much larger, more yellow) - it's some sort of ichneumon.
I think charlieb has taken this one as far as it is possible to go from a photo, as far as I'm aware most ichneumons are very hard to identify with certainty, and although there are an increasing number of named photos on the web I'm not convinced of the reliability of the identifications - they may be correct, but with few published keys available it is difficult to know how the names have been arrived at. Having said that, a real expert in the field who has studied enough specimens could no doubt go much further with these than I can.
Entomologist and biological recorder