Its a fairly regular thing.
When I lived near Heathrow we used to get a twin prop come over in the early hours of the morning checking the ILS etc. I think it was about twice a year or so, it even made approaches to runway 23 which was not ILS equipped.

Theres also Flight Calibration Services ltd. I photographed G-FCSL at Cranfield back in 2008 and that was used to test the ILS etc.