Thank you all for the suggestions of products to try. Here's a list of the things suggested and whether I've tried them or not:
Haven't tried
body shop ginger shampoo
Philip Kingley’s scalp tonic
Neutrogena TGel with tar ingredient
L'Oréal elvive Phytoclear shampoo- normal and sensitive
Head and shoulders clinical solutions with selenium
Head and shoulders argan range (I have a severe nut allergy, so I'm reluctant to try anything Argan nut derived)
Boots sensitive shampoo
L'Oréal Lo Shampoo
Allergenics
Aspirin (this is an interesting one!)
Miconazole (I have tried other antifungal medicated shampoos)
La Roche-Posay Kerium DS Anti-Dandruff Intensive
Green People for Men Itch Away Shampoo
Tried and didn't seem to work
Cocolis
Oilatum
Nizoral (but I did use this successfully for years for less severe 'dandruff')
Betnovate scalp
Moogoo Shampoo
Dandrazol Shampoo
Tried and seemed to help a bit but not clear it
Capasal
Dermax
Soaking in olive oil then easing the flakes out (but I did this about 6 weeks ago and the regrowth has been so vicious this time)
Things not mentioned above that I've previously tried that haven't worked
Yes to Carrots
Jason shampoo
Neem shampoo
Avalon Organics Tea Tree Scalp shampoo
Bodyshop Rainforest shampoo
Eucerin Shampoo
To answer some of the questions here:
For each medicated shampoo prescribed by the GP I have used it as per the instructions for around 4–6 weeks before stopping if I've noticed no improvement.
I saw a dermatologist in February, when unfortunately I only had a small patch showing, so it wasn't the best use of the appointment. She diagnosed it as Sebhorreic Dermatitis and prescribed Dandrazol (Ketoconazole) shampoo plus a Steroid treatment which I can't remember the name of (it's at home and I'm away this week so I can't check). Neither seemed to work at all. It's the Dandrazol that I've been using these past couple of weeks while its gone from moderately bad to awful.
I've just been to the pharmacy where I'm staying and the strongest thing they could sell me OTC is Alphosyl which is 5% Coal Tar (5 times the amount in Capasal). I'll give that a go. I feel the need to try something heavily medicated as it's so bad, then I will try some of the things suggested here.
I have curly, dry hair. I've been using gentle shampoos and non-silicone conditioners since 2012 (this scalp problem pre-dates that so I don't think the gentle shampoos are the cause). I tend to wash my hair every other day with shampoo (medicated when scalp is bad, a gentle shampoo otherwise), then conditioner. I rinse my hair very very thoroughly as I suspect conditioner won't help my scalp. Then I use boots curl cream but I'm careful not to get this on my scalp.
Info about my health: I am atopic (asthma, eczema, food allergies). My eczema on my body is well controlled by Eucerin 10% Urea lotion and by avoiding wearing synthetic fabrics, avoiding fabric softeners and avoiding certain soaps/shower gels. My asthma is well controlled using Symbicort. I have Ulcerative Colitis but it is currently in remission without medication. I have previously needed immunosuppressants for this but last took them in 2015.
The only time in the past 20 years that my scalp has completely cleared was during pregnancy.