The doctor confirmed that the issue with my eye is scleritis, even though an optician and a supposed specialist didn't think so before.
Why I've got it, and why it should have come back, is another matter. It can apparently be linked to rheumatoid arthritis and certain inflammatory bowel conditions. The latter would seem the more likely, considering my long term struggle with IBS and previous check for Crohn's disease, so she was asking plenty of questions in that regard.
I've been prescribed a plethora of drops and creams and got to go back in a week. If it keeps recurring then further exploration will need to be done to isolate the cause.