ok... thanks for all the advice, people! I think i've found the leak:
Land Rover Discovery 300Tdi Passenger Footwell Water Leak - YouTube
Underneath the washer bottle there are no leaves and when the vehicle is just standing in the rain no water seems to come in. When i'm driving on wet roads, however, that's when the water seems to come in.
do you guys agree that it's the grommet shown in the video or is there anything above there that it could be coming from?
I basically got it to leak by sticking the hosepipe pointing downwards in front of the washer bottle. It was on spray though, so water was basically going everywhere in that corner of the engine bay.
nick2303: thanks for the advice - do you agree it's the same grommet you had trouble with? If so, did you have to remove the grommet to clean all the rust off, or did you just basically clean around it or just lift it's edges, but leave it in place?
Regarding the list of stuff i'll need:
1 some rust treatment... -
I have Waxoyl aerosol... presumably this is not ideal - I need something that can be painted or sprayed on? Will anything from Halfords do? Or Homebase/B&Q
2 some silicon sealer.... -
Got it.
3 paint.. -
Why? to make it look nice? or to protect it? I have some black external Hammerite - I guess that'll do.
4 paint brush.. -
Got loads.
5 a wet finger to spread the silicon sealer.. -
Sure I can get one of them!
6 a cloth to wipe yr finger after youve spread the silicon sealer.. -
Pair of jeans.
7 a bin to put the cloth in after you have wiped the silicon sealer off ya finger.. -
Yeah, yeah...
8 a bin man to empty the bin.... etc...etc... ad infinitum..... -
Oh, but what if the bin man won't empty the bin? Do i phone the council? Should i dispose of the rag correctly? What class of "disposed waste" would it be? 
