Hi Graeme...welcome to the forum
Sorry for the late reply, only just read this post.
I would try a cleaner first, especially if the car is only smoked around town.
Firstly, I believe the fault is with a lazy sensor so a cleaner is a good place to start. I have previously used Halfords own cleaner as it looked the most aggressive with the most amount of Ethanol in it, along with an Italian Tune-up, worked a treat.
Let us know how you get on with it
Cheers Trevor