Front door seal perishing

Looks like I may need to replace the drivers side inner door seal. It looks like over the years when getting in and out of the car the seal has been rubbed against and is looking a bit worse for wear, its not leaking (YET ) but doesn't look good.

Has anyone replaced this before and can this been done easily. I fear this may be an expensive part, as I recall having to do this on a mkIV Golf and it costing over £100!

I may be able to get a good seal from Skospares in Poole but its the refitting part that I'm not sure on as if it goes wrong I may get some leakage.