Where is the fuel shut off valve on my C5 HPI?

I have a C5 which like all of them suffers from the occastional anti pollution fault. But now it has started cutting out at high speed. The local dealership cant seem to fix it but I am suspicous that the fuel pressure is the problem. I know that the pressure sensor is located in the cylinder head but is the fuel shut off valve there as well?
I will inspect and replace these as a start before looking at the fuel pump.