Rover 25 starting issue

The Rover 25 will not start unless I hold the Accelerator circa halfway down. Then it starts but would cut-out if I did not move away immediately. It then runs fine and ticks over fine at Traffic Lights etc. I noticed the other day when I was putting Petrol in the Car that when I removed the Fuel Cap the Tank "gasped" as if short of Air. If the vent pipe was partially clogged and thus causing a vacuum would this account for the starting issue. Does anyone know where the Vent pipe is located and should I check same.