I've spoke to Epson (they have a live online chat) and they suggested Epson Archival Matte Paper. It much heavier than what I'm using and should make the K3 Ink appear better on paper.
It can be found towards the bottom of this page
Epson Paper
It's around £35 for 50 A3 sheets which is 300x more than what I paid for my current 'cheap' paper (£10 for 500 sheets!)
I really want a quality print though; this printer is the best in its league and has superior ink technology. I could always use the cheaper paper for development work, then use more expensive for pin-up and presentation etc.
I will take a look at HP too. (Epson said the Archival Matte paper is also bright white, unlike their premium glossy which is slightly off-white)