Shower mixer cartridge

I had to do exactly the same job last week on my bailey Orion. I'm a bit concerned about the plastic nut you mention ? . Once I'd removed the handle then undid the cowl there was a brass nut , took that off and just lifted cartridge out . Wondering if your shower tap isn't original. If the cartridge is 35mil diameter , you can pick them up at Screwfix for less than a fiver . Good luck.

van is only 8 month old and yes ive seen metal nuts before 

never seen a plastic nut and recessed so will be a bugger to get out 

Armourer . If you are not intending to use the van any time soon I'd get it straight back to your supplier .There's no way a tap cartridge should be screwed in a van that's only 8 months old , has to be a warranty job and let them have the hassle . I'd get them to replace the other existing cartridges as well . Be on the safe side.