Is it a hole that's supposed to be there or a rust hole ? It could be a drain hole which if it's letting water in means it's blocked somewhere, siliconing it will make it worse (well it won't make it betterrer).

Get a long piece of wire and have a poke around in it, see if you can blast some water into it under pressure and if you can find where the exit of the drain hole is and do the wire poke in there. The exit is where the blockage is most likely.

Found this which seems to cover most leaks on a Mondeo ;

http://www.mirez.co.uk/WaterProbs.htm

It's definitely not a drain hole, it's a sharp corner that has either rusted or not been sealed properly before. The other side is fine. I'll post a picture of it later. The water is supposed to run past this tiny hole down the side of the boot along the drain channels to the back of the car and down to the floor.

Yep, but given the current climate and increasing number of scenarios where people doing perfectly legal things are being detained as terrorists, it wouldn't be worth the potential bother.

Yet.

Very true.

Some pics of my leaky car bits.

Junk in the trunk? A big box full of stuff you don't really need.

Literally yes.  Or a fat ass on a woman. 

OM/G/!!! The water is coming out somewhere /else/ now. Less than before so we're obviously doing something right. All in the back left corner of the boot. Grrrrrrr.

Right. Well, I removed the light cluster, tried to find a leaky spot. No luck. Put the light back in so I could spray the car with more water and... nothing. Not a drop and loads of water on it. I really really really hope that may be it for the boot.

However, there is also water in the front passenger foot well which, from the link posted about, is probably the pollen filter leaking. Another thing to fix.

If it keeps up seal the trunk up real good and buy some fish and turtles. 

Haha. I think that might be a good idea. Turn it onto a fish tank/pond.

I'm still waiting for some proper rainy time but there have been a couple of days of heavy rain this week and we seem to only have about a tea spoon of water in the boot now. Much better than the pond we used to have. No horrible damp smell in there now either.