Have read a lot about this theory (much of it by the arch-conspiracy theorist, David Icke, but to be fair, several people sem to agree with him this time!)
The theory is basically as follows. The MI5/MI6/Prince Phillip, the Duke of Edinburgh (or perm any 1, 2 or 3 of them in any combination, according to who you believe or don`t) didn`t want Diana to marry Dodi Fayed because any children they subsequently had would be half Muslim, thus if they ever became heirs to the throne the Anglican link dating back to Henry VIII would be broken. So they engineered a car crash, by (again, you take your choice, I`ve heard all of these suggested:) tampering with the car brakes; spiking the chauffeur`s drink; having a driver coming in the opposite direction flashing a huge spotlight into the chauffeur`s face to temporarily blind him and make him lose control of the car; having a white Fiat Uno clip the royal Mercedes just sufficiently to make it lose control and hit the post at very high speed; etc etc.
Oh, yes, and this can allegedly be `proved` by unexplained amounts of money being paid into the chauffeur`s bank accounts just before the `accident`; and by the fact that only 1 occupant (who I presume must have been working in cohorts with the assassins) was wearing a seat-belt at the time of impact, contrary to regulations - apparently he de-activated the seat-belts of the other 3 by remote control. Diana herself was `seen` by some in a distressed state, wandering about in the tunnel. And so on.....
Any ideas or comments or extra info. most welcome!