Have seats with lapbelts in our ducato panel van want to put third point in the C pillar. C pillar seems very light was wondering if anyone has had them done and what the reinforcement was.

I'm tempted to put a plate all the way from the longitudinal roof member to the midrail on floor but my entire family are known for total overkill in the belt and braces department.

I can't answer your specific quesion, but the standard reinforcing plate is 2" x 2" for seat belt mounts. Have a look at the securon website for examples.
Part Number 681/4

Cheers I put in a plate about that size but the pillar is so thin that a good tug will flex it. I've seen the tester swing his full weight off the belts so was thinking if the engineer saw movement he wouldn't sign it off.

I've an (older) car where the seat belt mounts go though the floorpan, if you tug them you could distort the floorpan upwards. You'd never pull the reinforcing plate out though, and that's a similar size.

I've never found any great detail on fixing points, and how much support is required around them, apart from the above.
I'm building a kit car I need to IVA, and I think I'm going to face the same problem.

The roof hasn't any great support in it, the danger of plating the c pillar is you may be the only thing still stilling upright if the roof is pushed down, which would be bad.
It may be better to ue a larger plate, but stop short of the top and bottom of the pillar, just to reduce the visible flex.