A curious insect has been posted on iSpot which you might like to look at:
It appears to have a wasp waist and looks too robust for an ichneumonoid. However it has very long antennae and an abdomen very dark dorsally and pale ventrally, which looks like nothing I can find in your records or random Googling for a possible vagrant. Oh, and it's from Orkney, which doesn't strike me as the most species-diverse area (or a likely place for a vagrant). Baffled.