Thanks Bigtum. My problem is that the boat is too wide for the Renault bars - I need them to stick out at the sides. The bars themselves are not worth very much, I already have the old feet and fitting kits for sale on fleabay. I could buy the Renault bars and extend them with bits of wood or something, but aesthetically that would be worse than the gappy fit of the Thule system. I'm considering some cheapo ones which I might be able to fit to the old bars, or bidding on 751 + 3078 bits.
I see your predicament Ian. My initial reason for choosing the Thule option was so that I could fit a box plus two bikes, but for me, aesthetics were an important consideration also, thus, the bikes were sacrificed (or at least one of them): I think in your position, the aesthetics may have to come second place to practicalities.
You are of course perfectly right in dissmissing the Renault bars for your intended purpose: try the cheaper alternatives or Thule (as you have already suggested).
Hope you manage to find a solution you are satisfied with.