Missing idler pulley

[Shaky]

I have just found that my idler pulley on my alternator belt has gone missing.  Its the one just next to the tensioner. I presume its flat (as its on the back side of the belt), and about 50mm in diameter. I assume its plastic as they often are. The engine is a Sigma 1.6 but a good source has told me that, although its a standard pulley, its not likely to be the one Ford would have fitted. I'll try Caterham on Monday but as I'm due at a track day on Wednesday I want to get ahead of the game trying to find out what it is. Any thoughts.