I have the same problem and a work colleague has found the right relay online in America but looking for a UK stockist at the moment - I'll post again when I know more
However I have noticed that, in my instance anyway, by undoing the screws holding the bottom section of the boiler front up and allowing it to drop forward until it lays flat, the relay has stopped chattering. Is a quick fix for me at the moment anyway
Tony C
January 2009