I went to see Dr Fenton (Harley St Clinic), who's considered to be THE dermatologist for hair problems. It's expensive but if you live in London you should be able to get a referral to his free NHS clinic at Guy's and St Thomas's. Basically what they will do is rule out any underlying cause and check your levels of various nutrients (or ask your GP to do this - though there are 2–3 that GPs can't do). You do need to be at the top end for each one when it comes to hair growth, as other PPs have said.
If there is no underlying cause – i.e. it's genetic – then taking supplements, tailored to your needs, and applying Minoxidil is the only answer. You need to take the 5% Minoxidil (Regaine) even though it says it's for men only. The Minoxidil liquid can irritate the scalp, but you can buy the foam, which doesn't. The foam is messy to apply, but you can put the dose in the cap and then plonk the cap in hot water in the sink for a minute or so. This melts the foam to a liquid that you can then apply using the dropper.
Hair loss is miserable and has plagued me for all my adult life (my mother's and grandmother's was severe, so when it started, I knew I was heading in the same direction). A proper assessment from Dr Fenton, Minoxidil and the correct supplements have saved me – my hair has thinned but it is nowhere near as bad as my mother's and grandmother's and no one has noticed (or at least, enough people tell me I have good hair that I THINK no one has noticed). I bulk it up with thickening shampoos (Philip Kingsley, John Frieda and Cowshed are good) and have regular highlights.
No shampoo, potion, lotion, herb, whatever, will make your hair grow - whatever people say. Minoxidil is the only thing that works for women currently. Some people swear that a shampoo has made a difference; I can only imagine that they only had temporary shedding anyway, and the shampoo use coincided with the regrowth.