Thanks to all for your illustrations and help here. My sister's 2001 E39 seat cover broke the other week and I bought the covers and associated stuff from Pelican Parts but the trim removal tools from my local Snap On Tools supplier. Thanks to what you have here, I followed the instructions and it took me 30 minutes to remove and install the new one without any dramas.
The only tricky bit was tring to locate the torn screw on the top of the seat towards the rear. Otherwise all good and it looks great.
Thanks again all.