Trouble Running On Idle & Running Rich

[giblet]

Evening chaps. Slight issue with my car, both yesterday and today the car was struggling on start up after having being parked up overnight. It started and cut out yesterday and today it was close to doing the same. It also seems to be running much richer if the smell of unburnt fuel is anything to go by.

Looking at fuel usage it has dropped by 7mpg between the last two tanks. First tank was the motorway miles to Mr Mann's place and back with some WOT thrown in at times. Second tank was Leeds to the Lake District with a bit more WOT this time but still a fairly similar driving style.

Could the lambda sensor be on its way out? Its the only thing that has been touched in between tanks that I can think of that could be causing the issue.