So with just a bit of rain today I managed to catch it at just the right time to see where exactly this leak first starts coming in. It's about 10cm left of the speaker when the door is shut (although in this image it looks like the speaker lines up with the leak).
I took the speaker off but it was all bone dry in there, although I'd need more rain to be sure it isn't coming out of here. I'm feeling a bit guilty now I've just butchered my seal a bit too much so I may have just made things worse I'll have to wait and see!
I've only recently started trying to do things myself with the car, I've got a Haynes manual on the way (only cost me £3!) so hopefully this will show me how to take a door card off and I might find it easier than referring to YouTube as I tend to get distracted!