"Unfortunately I am very allergic to vitamin C so that's not an option for me. It's weird because this is something I had never realised until I was tested for it last year, but even when very fit and well would have an allergic reaction to eating oranges, so I just didn't eat them. "

I can't see how anyone can be allergic to vitamin c. Allergy to citrus is pretty common, but not an allergy to vitamin c itself. There are forms of vitamin c that are not from citrus. You might even want to look for synthesized vitamin c that is not even from a plant source. It is typically the cheapest vitamin c available.