Curry leaves come from the plant Murraya koenigii. They are a little like bay leaves but are not as pungent and impart an earthy taste to curries. You can buy them from http://www.curryhouse.co.uk/cw/index.htm
You can buy then in Asda (battersea) believe it or not; they call them Methi leaves but I'm willing to bet this is what you mean. Plus there IS a plant called the curry plant cos I've grown it so yah boo to whoever said there's no such thing.
Hamish, sorry but there is such a thing.
You can buy fresh ones at the spice shop in Portobello Road or through their website www.thespiceshop.co.uk - some larger Sainsbury's stock them, too (in the Jamie Oliver range of fresh herbs and spices!)
My appologies. Whilest curry is itself a mix of spices it seems to have lent its name to one of said spices. I found this on the Oxford Dictionary site: b. attrib. and Comb., as curry-sauce, -stuff; curry-leaf tree, a name for Bergera K�nigii, the aromatic leaves of which are used to flavour curries; curry-paste, -powder, preparations of turmeric and strong spices, for making curried dishes. Thanks, Hamish