Cheers Dean , just taken a closer look at this.. worth looking at the comcar site.
My understanding the issue with the 18's is on the estates, they are required if the tow bar is ordered. The Co2 on the premium/ premium plus estates is slightly raised over the saloon at 153 verses 148 which affects the tax at 9% instead of 5% for the saloon. Neither model's Co2 is listed separately with 17's or 18's fitted
To note, the sport estate is 148, adding the premium and premium plus packs increases the Co2 to 153 for the estates. The saloons stay the same regardless of spec.
http://comcar.co.uk/ The government grant is payed on the basis that the car is a plug in and can do more than 10 miles before it goes flat rather than Co2 levels.
https://www.gov.uk/government/publications/plug-in-car-grant/plug-in-car-grant-vehicles
I didn't go for the estate because as you say it came out £75-£80 a month more than the saloon on the on line quote system.
As I've ordered a saloon I shouldn't be affected by the spec wheels etc. Hope this makes sense please feel free to correct if its amiss as I don't want to be caught out with unexpected bills.