Thanks everyone for your input, especially Eurajohn. I've now had some clarification from the Rapido factory, they state that a roofrack cannot be fitted to the 896F because the roof is entirely styrofoam. The dealer (in Bordeaux) had told be that a roofrack could be fitted and I've been asking them for 2 months where the mounting points are, what is below the plastic substrate, etc., etc., and they have sent me lots of photos of the roof, factory schematics (which showed nothing but peripheral lines and wiring tracks) and lots of other useless info, all the time it appears that they didn't really know the facts but just wanted a sale.
I'm a former motorhome builder / van converter and I know that there's a couple of options which I could take. One of the solutions would be that described by Eurajohn but having discovered that the outer skin is not aluminium or fiberglass sheet, just a plastic composite I decided that would not be appropriate in this case as the roofrack is only ever going to be as strong as the surface it's attached to. Hence a surface mount similar to Eurajohn's incorperating a through-mount and internal reinforcement is the only safe option, but that would invalidate any warranty on the bodyshell. Therefore, I've decided to walk away from it....it's a lovely 'van at a great price, and the dealer is not at all happy! But it just goes to show what some dealers will do just to get a sale, and that some may talk (very convincingly) as if they know the product well, when in fact they know little more than you or I.