There is a MiG-21 at Boscombe in the museum. It does get moved out of HAS now and again. This Slovak MiG-21MF flew into the UK during 1994 and was presented to the RAF Benvolent Fund. Does it fit in with your time frame?
It was probably 2004/5 that I thought I saw it, your post may have just put this in my memory but I think I remember it being quite close a HAS and was with a few other period fighters as well. This would have been on the west side of the field, going along the road that leads to Porton.