LongtoothedGran, you have to decide what's right for your own piece of mind.
I have been told categorically neither to attend a doctor's surgery nor hospital, except in an emergency, which is what happened to me last week, an emergency admission. All my treatment and scans have been postponed during my continued shielding, that also includes a scan of my hip and a coronary angiogram that are needed.
The clinician who asked you to shield, must be aware that you need a scan, has he asked you not to attend? If not, does that mean the need for the scan overrides the risk? Just playing Devil's advocate here.
What also occurred to me was that if you decide to cancel, you may have to wait a considerable time for another appointment, due to the backlog caused by Covid. I suppose it's a matter of weighing up how your problem is affecting your life and how anxious you feel. It's about balancing the pros and cons. I don't envy you, the decision was made for me by my consultant.
Anyway, I wish you the best of luck, it's not an easy decision. I don't envy you.