Thanks for the reply. I should have said I'd tried there for those dates because although all these places say they cover the parish for marriages at that time, in reality they don't (or rather they do but you can't search them properly).
I think I've solved it though - on Family Search, I found the digitised records for that period, which have been attributed to the non-existent parish of Clemenwastand
I admit the handwriting is bad but you'd think the transcriber would check the names of Norfolk parishes. This error appears to have been copied across several family history sites, which is frustrating but I've found the marriage I was looking for so all's well etc.
Edited to add that now I have all the details of the marriage, I can find it on Ancestry in the Norfolk, England, Transcripts of Church of England Baptism, Marriage and Burial Registers, 1600-1935, which I couldn't find before in there.