Well, if you have the time - here is a recipe:
http://www.cupcakeproject.com/2009/09/clotted-cream-recipe-making-clotted.html
... or you could just stick with double cream from Gruyere.
And of course, we can always rely on Delia who writes:
Clotted cream: Wait for it - this is the big one! Perhaps you'd rather not know, but it has at least 55 per cent butterfat. Clotted cream has a unique and special dairy colour, like pale buttercups, and is thick, rich and utterly irresistible. It is a speciality of the rich pastureland of the West Country, and is made by heating the cream to evaporate some of the liquids, so, in a sense, you could call it concentrated cream. It is heaven spread on scones with home-made preserves and extra special on tart fruit pies. It's not for every day, but everyone should treat themselves to some just once in a while.