BBC news item It seems to have been a Londoner, Nicholas Milligan, and his family. No other water users/craft reportedly involved, until a ski instructor managed to get aboard and bring the boat under control.
The survivors sound to be in a pretty bad way also, two dead is too many anyway, but one hopes that there will be no more.
Is there a definition which rules out RIBs from being called speed boats? Some of them are seriously fast, outboard-powered or not, and the pictures show a pretty big engine hanging off the stern of that one.