Pleased to say that I found the species at another site today, the disused railway from Cardington to Old Warden Tunnel. It was found at two locations in different 10km squares, so that's three new 10km squares added in the last eight days.
There was one pair amongst them:
This shows what they look like in context. Easy to find sitting on top of foliage, usually near to wild liquorice rather than on it, as here. They seem to sit out much of the day like this.