Thank you so much for the detailed response, I really appreciate it. Over the years I've looked into this a lot but you've given me some really useful new information!
Health care in the UK, especially for lesser known genetic diseases, can be a bit of a lottery... I moved up the country 6 months ago, and within a month had been tested & diagnosed for a generic mutation called FMF (familial Mediterranean fever). My dad / sister both have it but despite nearly a decade of requests I was unable to get a doc to investigate it. So far up here the gp response has been a referral to a website for cognitive behavioural therapy.
I'll push on though and see if there's anything more they can investigate. Thanks again for the info :)