Thanks for the suggestions, however I have sort of solved it. Out of curiosity I tried some old subright paper I had from before I switched from sawgrass to cityinkexpress ink and the prints came out 100% spot on, no white lines and no blotchy patches.
Odd as I was halfway through a batch of paper when the problem occurred, so I tried a fresh batch of city ink paper and the problem returned.
In regards to CIE being cheap, it's actually produced better results for me than sawgrass inks ever did, their support on the 2 occasions I've previously needed it has been spot on as well. I've emailed them today to ask about the paper.
I'll keep this updated in case anyone has the same problem