I agree with stevie. Try the Neutrogena T-Gel Therapeutic shampoo. I used to use T-Gel Anti-Residue shampoo for its cleansing properties and it was brilliant. I've also used the Therapeutic one occasionally just to keep any scalp conditions at bay.
The Therapeutic one is very good although it does smell a bit medicinal and is pricey. Once your dandruff is gone you should still use it once a fortnight.
Thanks to you all who responded. I will give them all ago,starting with the tesco's one. I am in there nearly every day.
I was told by an old irish barber, when i ask'ed his opinion, that fairy liquid was as good as anything.
My son in laws are mainly bald so i don't bring the subject up with them. Diplomacy !!!. Thanks everyone.