Hello, I have a 2015 Mk3.5 focus Zetec s which is current running 18x8j alloys, I'm looking to change the alloys, would I be safe running an 18x9j alloy with a 235/40/18 tyre? Or would I be better going for the 8j option? Tia

Whether it fits or not fully depends on the offset of the wheel.

The stock 18 Inch rims are 8 Inch wide and have an offset of 55 mm. A 9 Inch wide rim with the same offset will be half an Inch wider at both the inside and the outside compared to the stock rim rim. To keep the same clearance between the suspension strut and the rim the 9 Inch wide rim needs to have a lower offset. If you want to install 9 Inch wide wheels I recommend to use wheels with an offset of 45 mm or 43 mm. However you must be aware that in this case the wheel will be positioned approximately 1 Inch further to the outside compared with the stock wheel.

Another point of concern is the width of the tyre. Based on good practise a 235 mm wide tyre is the bare minimum width that can be installed on a 9 Inch wide rim (235 to 265 mm wide tyres can be installed on a 9 Inch wide rim where 245 and 255 mm wide tyres are considered to be the best choice). When installing a 235 mm wide tyre on a 9 Inch wide rim the rim will be flush with the side of the tyre or even protrude. In this case there is a very high risk for kerb damage. 

In my opinion You should stick with 8 Inch wide rims or alternatively install 8.5 Inch wide rims.

Thank you very much for your reply, I'll go with the 8inch