Thanks for the reply Jean-Luc.
I've just received both the amp and footswitch, so I don't know if it's worked before.
Yes, it's the FS-4 which is labelled as you say.
The cable is hard-wired in the footswitch rather than in a 1/4" socket, but I am connecting it to the correct socket on the rear of the amp. With the cable being hard-wired I cannot change the cable.
I might have to open up the FS-4 to check inside the unit, but I'm not sure what I'm looking for (unless it's an obvious fix)