No, I didn't try the reset, what has cleared it was linking it to my iMac.
iMac wanted to download all the photos (already there though) so I declined, but when it offered to delete the photos on the phone I said Yes.
Result is that Camera Roll is empty, but My Photo Stream still has all the photos.
Everything seems to be back to normal. I can take a photo, it appears in Camera Roll, and I can then view it and delete it normally.
Puzzles me what happened in the first place though??