Many thanks everybody.
Thanks to all your help I think the mystery is solved.
After billybirder and bigdave60dog suggesting hen harrier I looked at harriers online and we think it was a marsh harrier.
Apparently they do occur in Norfolk and I've watched a few videos and it seems to fit the bill.
We had been out this afternoon to a local water garden where we expected to see overwintering water birds and where we had seen a kingfisher before.
We had a lovely (if rather bracing) walk and saw some red deer hinds. But no birds apart from what we get on our feeders in the garden.
I'd put the camera away and were only about 300 yards from home when we saw this.
I'll keep it handy until I'm indoors next time

Thanks again