Hi Andrew.
I got a 3rd rail retro-fitted to my T5.1 4mo by Exploria in Brighouse- they gave a good service, doing a good job. There’s a spot in Oxford I think that will do it too (I think Exploria quoted me less).
Bear in mind that you must make sure your insurance company understands the additional seat scenario and that you’re covered for 5 people, because otherwise they’ll go off your log book, which I bet it says 4 seats (so you could void your insurance if you drive with five…).
The 5th seat is a great option- IF YOU NEED IT (judging by the amount of 5th seats for sale, I imagine a lot people regret paying for the option).
Don’t pay anything above £150 for a second hand one though (saw one recently up for £750 ).
Lastly, another consideration is the weight factor; 5th seats weigh 40kgs (I think) on their own, so they’re not easy to lug around, and if - for instance - you have a Cali with a 4mo drivetrain, 5 big adults, loads of gear, bikes & gubbins, and you exceed the 3 tonne gross vehicle weight, and have an accident- you’ve probably voided your insurance too…
And you’re fkd.
We used ours recently with 3 adults and 2 Isofixed babies, and it worked a treat.
Enjoy.