Two reasons have come up on here before:
- If the ECU is "re-learning" after a long lay off: I've no idea why this would be, but it may be possible.
- Hose into the fuel filter perishes: Take a sniff in the rear wheelarch (offside, IIRC). If it's worse there, look to change the filter and check the hoses.
And as Ziggy says, have a sniff around the engine bay, just in case. As long as it's not too windy, and you have a half-functional schnozz, you should be able to get a decent idea where the smell is coming from.