conditioner dispenser doesn't fill with water?

I have a bosch wfl246kgb/04 washing machine and the conditioner drawer does not fill up with water on the final rinse cycle, preventing the liquid to syphon out into the drum. When the fault started to occur, it was just that there was not enough water being put in, now there is none at all. I suspect that it's because of the 2-part solenoid valve on the cold feed not operating but can't see how to confirm the fault, short of just replacing it at a cost of approx £20. Any ideas?