Bongo Fury

I have had a rear conversion completed in my bongo.I have 5 seatbelted seats,The other day i needed to transport 5 other people,if someone sits on one of the side seats at the back is it legal(i know not recommended) ? I can i fit a seat belt to make them secure but is this legal?

Firstly from memory side ways seats are rarely fitted with belts, Discoverys are the only one I can think of
I would have a chat with Securon the seat belt people for some advice