Got me this off a chap on 22b...
Genuine Prodrive part, apparently designed to stop power steering fluid boiling in the WRC Cars and also eliminates use of Power steering cooler, weighs next to nothing and looks like it took a bit of skill to make!
Will need to get some sort of bracket made up to fit it I think as it seems it was designed to fit up by the bulk head Plus I have no brackets on new intake manifold so will find somewhere suitable to mount it
Picture of tank up at top right of engine bay