I am very confused by this insect. I thought when taking the photos that it was a Symphata, because of the thick abdomen at the base. But the head is round - the wrong shape for any Symphata I have seen. But I have found that Sapyga quinquepuntata looks quite similar, except that it has black legs, not orange. and mine has white marks at the base of the tibias. There is no other species in that genus in the UK.
(my last image has one of the legs crossed over underneath, so it looks like it has 4 legs on that side!!!).
I hope I have put this hymenoptera in the right forum!
Size - a bit more than 10 mm.
Date: 11th June.
Location: Norfolk, England.
Habitat - marshland at sea level, but mainly freshwater marsh.