Hi Coz,
I think that pipe will go back to one of the cylindrical canisters on the right side of the engine bay - I think it is to stop air getting trapped at the top of the rad. I remember seeing it in a diagram in the factor owner's handbook. Hope that helps (sorry it's a little vague - I'm a couple of miles from my handbook and workshop manual). I'll have a look when I get home, and let you know (unless someone else has already...)
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