Stuart,
Remove the screws holding the inner frame in the door and then take the frame out. Wind the window down and free the channel from the door frame, starting top centre and working round each way. Finally pull it up at the ends. You may need to remove the door card to access the bottom of the channel where each end is secured by a bent steel tag at the bottom of the metal channel frame.
You will certainly have to access that area when fitting the new channel so it is probably worth taking the door card off from the start.
You will need to either transfer or replace the metal clips that hold the channel in position in the upper part of the frame. I would suggest replacement – If the Club Spares does not have them they are available from CTA – see: http://www.ctaservice.nl/product/21966/klemset-raamloper-bevestiging
If possible, cut the channel slightly longer (say 50mm) than required. Fit the clips then feed the widow channel down each side between the glass and the metal channel it sits in – you may need to wind the window up and down a bit to encourage it. Once the top of the new channel is located and held by the clips, wind the window up to hold it all tightly in place and then cut the bottom of the channel to length and ensure both ends are securely held by the those bent steel tags.
If any tags are not located properly (or broken), the channel will tend to be dragged down as the window is opened. If any are broken a small self-tapping screw may be necessary.
Good luck.
B……………….
Stuart Dicken
Bernie