There are several different ways of modifying the seats, rails & runners to give you the desired effect. Madmile has kindly put together a guide HERE to modifying the runners so that the seats can be fitted either way on them & still slide & lock both ways.
This is how Colin Lambert had his arranged.

Factsheets on reversing the seats here
Bongo Clinic Archives here.
Past & present discussions on the form can be found here:
Reversing seats
Removing seats
More information and photos of how it's done, from dvisor, ON THIS LINK