Easiest way to fix leak?

I just bought a 2002 Micra s last week and drove it from Belfast to Luton with no problem apart from stiff gears.
However, since I got back, it has been pouring and there is water dripping in to the passenger footwell. I did open the bonnet and found leaves and debris which I cleared and this morning there was maybe 1/4 cupful water on the rubber mat. Seems to be coming in less than it was yesterday.
Dh not happy at all! I think it is a new problem as there was no damp smell in the car when I got it, and no water staining.
He is convinced it is a massive job to fix, I want to have a go myself if it is easy enough. I do know that the heater matrix was recently replaced, so could it be that there is a leak from there?
Can anyone give me the dummies' guide on what to do please? Step-by-step? Or a link to similar.
Im pretty sure guy on here called PollyP had this problem and fixed it. send him a msg im sure he will reply and help.
alternatively try to search the forum because im sure you are not the first one
I will PM him, thanks. I did look through lots of threads, but they say to remove wipers, and look under the scuttle board or something. I don't know if that is easy to do, or how to do it.
Mckenziecz thanks for pointing me in the right direction.
PollyP Thanks for the pics in your blog. Looks like you have solved the problem for us. There was no rubber seal around the scuttle panel clips at all and one clip was broken. Just trying to get Nissan to order the right clip for us now.