Thanks for the suggestions. Romanrob, I bought new water shedders, one for each front door as I have some stuff to do in the passenger side door as well as the drivers side door. They are around £45 each from Landrover. I just prefer to use original stuff where possible, if only to ensure a proper fit.
I had thought of making one up, out of plastic sheet, but there are some many lumps and bumps in the door itself that I decided not to bother.
The chisel bit was a bit of a desperate suggestion !!! I do actually have a set of nylon/plastic trim tools, one of which will do as a perfect scraper. The comment that you made about using a thinning agent had also occurred to me - that even with most of the butyl sealer removed I could end up with a gloopy mess. I will just have to try to remove as much as possible without resorting to thinners.
By the way, here are the part numbers, just in case you need them:
SHEDDER-FRONT DOOR WATER RIGHT HAND - EJD500600
SHEDDER-FRONT DOOR WATER LEFT HAND - EJD500610
I bought them in euros - €59.00 each.
Mad-as, there are an awful lot of lumpy bits when you look at the door after the original water shedder is removed. I think that to get the new membrane to stick on, without sticking up too much, I will have to remove as much of the old sealer as possible to try to prevent lumps under the new water shedder.