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Look at repaircare if you want a better solution. They have a lot of videos showing the process. I get through quite a bit of it repairing these kinds of problems but I have to really like someone to want to do it these days. Working on your knees in the damp is a painful experience.
I filled my own with some epoxy resin cement I had left over in a tin and it's worked a treat for years.
Remove the old filler. Route out the really fucked stuff. Drown the rest with hardener. Fill and then sand back. Going to fit a rain deflector to the door and a new seal.
Might as well knock the bloody house down