Back again,

I have noticed a small amount of water beading under my boot floor mat on the bare metal and around the spare tyre. This is only present on the passenger side. 

Had the passenger side rear light off and cannot see any leakage signs from there. Wondering what could be causing this? It looks almost like condensation.

 

 

Vents behind the rear bumper or the seal on the rear roof section I'd guess. Have you tried removing the rubber bungs from under the rear bumper and seeing if any water runs out? And if the spongy insulation which is in the void wet through.

Welcome to the club - i.e. the self-propelled colander club.  It's a feature that Ford have steadfastly incorporated into all focus models, and others too!  You will probably lose the will to live, striving for a dry interior; but good luck.

I've had the passenger rear side light and panelling trim off. Can't see any water pooling or feel anything of the such. 

My current thought having seen this is possible condensation radiating from the exhaust system as this runs under this side of the boot.

I've had water get into my Focus through the vents behind the rear lights, I sorted that out which was easier to do than expected. Then I had water dripping onto the load liner from the top of the tailgate, after further investigation it was one of the bolts which was holding the spoiler on to the top of the hatch, my Focus is a Zetec S, I checked the rest of the bolts and they were tight. Go on YouTube and type in Ford Focus leaks and you'll see plenty of videos about the vents leaking.