Hi Adrian - do you have the 3rd rail required for the extra seat? If yes, then there are regular sellers on here looking to sell them.
If no, then you are into looking for an installer of the 3rd rail, which will be a specialist job, but again some forumites have experience of this and a quick search will pick up some of those recommendations.
How hard / how much £££ depends on how many seat rails you have.
If you are lucky you will have 3 aluminium rails in the back - it was a paid for option on a T6.1. If you have those its just a case of finding a seat & sliding. it in.