On occasions, while at idle, etc, I can get a whiff of petrol while mounted, could it be my fuel cap vent? Its not very strong, very subtle, but I did a looky round to nothing. As to the vent, the tank is not vented with its own dedicated line, right, just the cap itself?

The cap vents in as the gas is going out. It will also vent out if the tank ever becomes pressurized,......not likely while riding.
I'd look for a gas leak.

At a stop with the engine idling, engine heat might rise and warm the tank, or sunshine might do the same. Either will cause the tank to vent air and gas vapor through the gas cap. California bikes have a dedicated vent line running to a charcoal cannister.

I long ago got into the habit of filling the tank on my bike at the last gas station on the way home, so they always sit with full fuel and are ready to ride. When I stop at home and leave the bike on the driveway for a bit, it is very usual for the tank to make a singing or whistling noise as engine heat and/or sunshine warm it. I think that noise comes from the vented fuel cap.