Can you post pic of it?
90% sure it's the cartridge and needs a new one.
The AL website has approved engineers local to you on it.
Approx cost.
Cart £80
Labour £60 + vat
Ok, so it's a 609 model and uses a cartridge like this.
http://www.plumbcenter.co.uk/wcsstore7.00.00.656/ExtendedSitesCatalogAssetStore/images/products/AssetPush/wol-web-proof-pdp-w/std.lang.all/14/73/D51473_wpdp.jpg
If you have a combi boiler, you want the pink version - A gravity system with tanks in roof/airing cupboard with cylinder, the grey one.
The engineers on AL website are mostly independent guys/companies like myself (i'm in London, and too far away for you..) who have been on AL training courses.
The £60+vat labour figure is what i would charge for 1st hour/call out with £25 p/h after that, but really should be done in the hour unless your stopcock doesn't work etc..
There is a little annoying washer right at the back of the valve - A pro will get that out and get new one in. A bodger will throw away the new one and tell you he's done it.
You need a special tool to get it out, and it can easily add 20 mins to the job (within the hour)
HTH