Help needed – unidentified aircraft near Boscombe Down, UK

Hello.

For the past week or so there has been a turboprop powered aircraft flying level circuits at about 6000 ft (perhaps) above Salisbury. This happens late afternoon and well into the night.

I saw it the other day and it was a dark (grey?) cross planform with a small tail and the wing intersecting the fuselage about midway. I could see nothing hanging off the wings and the fuselage was narrow (almost as thick as the wing) when viewed from underneath. The fuselage and wing seemed to be similar length.

Does anyone have any thoughts?