Mocal Cooler

[MCH]

you don't have to keep it, seen them fitted both with and without.

I think my exchanger is allowing oil into the water hence replacing.

You don't have to no bud, but its pretty important to do so. 
Remember its not really a oil cooler as such, rather a heat exchanger. 

Replace it if needed buddy, find them very cheap

Where?

I had a look but always a bit apprehensive over pattern stuff as had various items fail on me where as when I've spent the extra or waited for genuine its never failed.
£200 odd for a genuine one is a tad pricey