I'm on a sickie today which has given me the chance to have my SBS radar box on. Earlier today, there were two military "business jet"-sized aircraft in the area. One was indeed a BA HS125 CC3 with registration 7D620. The other aircraft, which does have winglets, was a Bombardier Sentinel R1 with the registration ZJ691.
I suggest you google these registrations for photographs and see which one is the one you observed.
SFS