Are we under attack??

An aircraft of unknown origin has been buzzing hereford(UK) for the last hour (2000-2100hrs). It has been cirlcing the city. It sounds like a single engined millitary aircraft with reheat and quite powerful engines?

Despite it being a clear night(i can see stars) I cannot see the lights of the aircraft. At a guess I would say that it is flying below 10K feet. I only guess this as we have quite a few civil airlines going over at 25-35K feet.

Should I be worried or are they defending the SAS base at credenhill?

Any ideas anyone?