Cheers guys,
I've got the housing off now, and in answer to my other question - yes it looks like the oil pressure sender must be removed first. There isn't enough space to wriggle it out with the sender sticking off the bottom so I've left the housing attached loosely while I get a 24mm open ended spanner.
I have the later style pressure relief valve.
http://944foot2thefloor.blogspot.com/2010/07/how-to-remove-and-replace-oprv-oil.htmlIf you look at this link the page author goes through the different types of valves and explains how to identify them.