Omalus auratus Family Chrysididae Ruby-tailed Wasp.
I think this is the correct one, but I found several that look similar.
http://www.gwydir.demon.co.uk/insects/wasps.htm
Omalus aeneus
http://www.bwars.com/Omalus_aeneus.htm
Chrysis ignita is close too.
http://www.gardensafari.net/english/picpages/chrysis_ignita.htm
The pic in the gallery looks to have a different colour under the abdomen, but really the two species look alike!
http://www.gardensafari.net/english/remaining_wasps.htm
There are susbspecies of Omalus auratus
http://www.em.ipw.agrl.ethz.ch/types.php?key=Chrysididae
Mine's probably a male, females are mostly concave under the abdomen
http://www.earthlife.net/insects/hymenop.html#The%20Ruby-tailed%20Wasps
I found a site with pics!
http://www.chrysis.net/chrysis/gallery/photo.php?beg=3
This one shows that different colour underneath the abdomen of Chrysis ignita mine doesn't have that.
http://www.chrysis.net/chrysis/gallery/photo.php?beg=26
The abdomen shape of Philoctetes sculpticollis is like mine!
http://www.chrysis.net/chrysis/gallery/photo.php?beg=50
http://www.chrysis.net/chrysis/gallery/photo.php?beg=49
Philoctetes sculpticollis looks closer than this Chrysis ignita
http://www.chrysis.net/chrysis/gallery/photo.php?beg=72
There's a few pages of photos, looking through them Philoctetes sculpticollis fits BUT a search doesn't recognise the name!
http://www.chrysis.net/chrysis/gallery/photos.php?init=0
http://www.chrysis.net/index_en.php
I found some on bioimages, there is a pic of Omalus auratus which has the abdomen shape mine has, and it is said to be common (first I've seen), back to square one! Their dead specimens lack colour.
http://www.bioimages.org.uk/html/P4/P44034.php
http://www.bioimages.org.uk/html/P4/P44036.php
I read on one of the links there are 20 species in Britain, bioimages lists 13.
