Mercedes SLC

[vinnie07]

Mercedes SLC (only 11K on the clock) has sporadically started smelling of petrol through the heater when it is turned on, this only seems to happen on about 50% of the drives but it is very strong when it does, fuel can also be smelt around the bonnet area when this happens.
Any idea's? Just seems so bizarre it isn't happening every time I run the car.
Thanks for reading