Hi, 

I think the fuse may have gone in the 12v socket or cigarette lighter (whatever it's called these days) now do I have to open the box in the passenger compartment or the engine bay and which number fuse is it? 

Car is fiesta 2018

Engine compartment fusebox. Fuse number 83 (20 Amp)

https://www.fordservicecontent.com/Ford_Content/vdirsnet/OwnerManual/Home/Content?variantid=6102&languageCode=en&countryCode=USA&Uid=G1893598&ProcUid=G1893599&userMarket=gbr&div=f&vFilteringEnabled=False

Thank you! 

Does this have to come off? 

Oh dear !!!  😧 you're not meant to unplug the cables.

The design of the fuse box is a bit confusing for those that have not seen it before. It is basically constructed upside down. Those cables are essentially plugged in to what is the back of the fuse block.

If you look at the four corners you will see tabs that can be released and the whole fusebox module lifts up and out. Since you have disconnected the total power to your car by unplugging the module then you will be presented with quite a lot of faults and warning lights when you first turn your ignition on. Most will clear just by operating the ignition on and off once or twice. There will be faults stored in memory that can only be cleared by using something like FORScan.  There is no need to worry as it won't have caused any harm. You will find that any preferences that you have set up in the main instrument cluster will have been lost and you will have to reset a few of them.

Good luck. If stuck come back and ask for help.

 

Should have said that now would be a very good time to disconnect the negative of the battery before doing any further work.