Fitting window into carrier on a 1993 Vogue SE.

We have a 1993 3.9 litre petrol/LPG Vogue SE.
The window carrier channel in the front nearside door rusted through and failed and we have obtained a replacement from Famous Four (next day service, excellent).
We’re having difficulty, however fitting the window into the carrier. There is a rubber strip which was folded into a U channel shape which goes/went between the bottom of the window and the carrier but the thickness of the window plus the rubber strip is so great that we can’t get it into the carrier.
Any suggestions or tips on how fit the window would be appreciated.

Although I've never 'done' your particular car, I've done many others, & all are similar.

With the glass out of the car, place on an old bit of carpet or similar, lower side up, ensure the glass, channel & rubber are clean, place the rubber (folded) over the bottom of the glass, apply washing-up liquid to the rubber (to contact area)offer channel/bar in place, then sometimes with the palm of the hand 'whack' it down into place, otherwise, a long block of wood & rubber mallet can be used.
If, for some reason, it HAS to be done whilst in the door, then place a suitable size wood support from the door bottom to the glass, so the glass is a few inches above the window opening, then the glass can be again pressed from above.
The 'washing-up' lubricant is important to ease the tight-fitting glass into the channel, it will soon dry out after.

That's great thanks Modelman, we were working along the right lines (albeit with the glass still inside the door) but didn't give it quite enough "whack".