Another trip to the doctor this morning saw me referred to an optician followed by another referral to the specialist unit at hospital. It can take a day or two to hear from them so I guess I'll be sitting in the dark until then.

That was actually the opticians advice: sit in the dark as much as possible, wear sunglasses if you've got them and use a warm compress over the eye if needed

They don't seem 100% sure what it is; iritis has been ruled out and the thinking is now about scleritis or another type of inflammation which will likely require a course of steroids to resolve.