What radiator would be good for an MK , running a cosworth turbo engine?

At the moment its a VW rad not sure what its out of but just looks on the small side

Most of the turbo applications seem to be custom ones. A Civic double core works, but it is high and barely fits under a standard nosecone.

The radiator isn't too bad providing its getting lots of air through it. You could fit a £300 custom job and still find it insufficient if not enough air is flowing through, or the air has no where to go. An oil cooler will help you no end. Not sure if you're running an intercooler too as that'll keep temps slightly lower. Or run a separate radiator purely for the cooling of the turbo. SausageArm did exactly that in his MAC#1. Really nice install under the rear panel.

Is it the alloy polo rad?

Not sure what it is,,

Its running a Cosworth 4x4 intercooler, ,, i have recently fitted a Mini Cooper S bonnet vent to direct air in to it.

Its also fitted with Water/Meth injection