Any advice on replacing the seal around the n/s front window.

easiest way is to take the window frame out, i think only 4 bolts hold it in, 2 on the latch end of the door and 2 on the hinge end, take off the lower window seal (you may have to take the door panel off) then the frame will lift up and out of the door, replace the seals, lube it all up with plenty of silicone and slide the frame back in, it'll be easier if you had a helper rather than struggle on your own as its a bit fiddly to get out and back in.

I've only ever done this on disco 2 but disco 1 is very similar

Fixed and done so many thanks Clutchdust. You were correct in that there are fixings on both the hinge and opening sides. Didnt see the large bolt at the top near the mirror though as it was behind the plastic seal. Let hope that the bloody thing now stays in place. Thanks once again for your help.