Sooo the answer to the question is YES, with some ever so slight modding. The seat belts I've fitted are mk3 anniversary edition in red.
The rear seat belts were pretty much a straight swap, only difference being on the rear of the belt retraction unit there is a locator pin that you need to remove and locate into centre of the of the unit. There's are three holes, and when I got the belts, the locator pin was offset to the unit and wouldn't marry up to the locator hole. After repositioning the pin, the retraction unit then just nicely slotted into place and bolted up.
The fronts required a bit of cutting of the internal bodywork, literally a cm2, to allow the belt tensioner to sit behind the rear door card, then again literally just bolts in. All that's left to do then is drill a hole in the internal bodywork and bolt in the tensioning device, which I should imagine is probably REALLY IMPORTANT to do so!
I will post some pics up soon and everything will become a whole lot clearer.
ALSO!!!.... thinking about it, I'm not sure if this would work in a 5 door model. The rears maybe, but I doubt the fronts would due to the tensioner!!