Bongo Fury

We have just bought a bongo with a side conversion and 2 x 3 point seat belts fitted to the 3/4 rock and roll bed. I would like to have an additional seat belt in the back (we have 2 children but would be useful if transporting friends on short jouneys). Has anyone else done this? Is it legal? I know it will be a bit of a squash but fine for kids i think?
If this isn't advisable how easy is it to fit an additional removable seat behind the passenger seat?
Thanks in advance for any information!

I used one of the original Bongo inertia reel belts for the middle seat on our full width metal slide bed conversion . Cannon Forge welded a special mounting bracket on the frame (didn't charge me any extra on the bed price) and it is very strong. Works a treat -takes three abreast -all inertia reel 3 point . I would never fit a lap belt.