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...
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