The seat belt buckle snagged on the drivers door aperture seal and ripped the seal. It appears that it just pulls out and you seat a new seal [may be a bit tricky getting it to align]. Is it just a press fit or is some sort of fixative or sealant used in installing the new seal? Other than the dealer or SNG Barratt is there another source in the USA that I should look at? According to Barratt the XR853720 part number has been superseded with part XR818114 [2005 S type ]

 

Is it the seal attached to the body or door...from your description i am thinking body...if it is the body seal, i believe it is a tight press in fit....i have the same issue and have located a couple of them on ebay....new and used...just have not pulled the trigger yet....have some rear bushings to replace first....my temporary fix is a piece of 3/8 inch foam filler rod and glue back in place...works nice...check this ebay item 271980058741

 

If it is not too bad you may be able to affect a repair of the rubber seal using super-glue.
I did this on my XJ40 when I could not get a replacement seal, it worked well and was vertualy invisiable.

Mellow

 

Now that it's above freezing I tried your 3/8" foam filler rod trick with some weather-strip glue I had laying around and presto - it works like a charm and is almost totally unnoticeable. Great advice!