Finally have some free time to start messing around again. I picked up some Kodak Ultra Endura. Managed a single sheet last night and so far I am optimistic the mottling may not be a problem anymore. Under-exposed but I'm getting a nice black, and no mottling is visible at the moment.
I still have to peg down ISO and filtration to be certain. I'll keep you all posted over the coming days or weeks.