Ok, so I recently bought a pair of Red Metal 3's and the front wheel and rims are great. However the rear hub wont freewheel properly.

At first it was uber stif (you had to put some effort in to turn it by hand). Thought it just neded to be lossened up with some riding. 50miles later, its loosened up a lot but it still doesnt freewheel properly.

The chain sags above the stay really too close to the wheel, and occassionally when pushing the bike in the car park etc the cranks go round rathe than the hub freewheeling (depending on what gear I'm in).

So my question is, is it a send em back job or does anyone know what the heck is going on and how to fix it?

Well. Im just going to strip 'er down and grease 'er up. Have a quick look myself. I think.

Were these bought new? If not then maybe someone serviced them and left a spacer out. If they are anything like Mavic freehubs I left a spacer out when I serviced a wheel and the freehub stuck just like yours.
The other thing is not to use grease inside the freehub or on the pawls and springs, just put some mineral oil in the freehub to keep it all lightly lubricated. Grease will create drag.

They are brand new, (well excluding the riding I've done on them).

Ahh didnt know that… bugger. So I shalln't be greasing her then…

I've seen one seize up completely. The freehub had to be replaced, but it wasn't expensive. The good news is that you can service them quite easily.

EDIT – I've just spotted you called it "her". I hereby withdraw my advice.

All bikes are female IME.