Someone upthread suggested exporting the KML of unvisited tiles from Veloviewer then converting it to GPX and importing into GPX Editor, which I have done.
From that, I put together this 150km route that bags 23 unvisited tiles and should increase my max cluster by >50 - was planning to ride it at the weekend, but didn't fancy going out in the biblical rain...
Hopefully will get the opportunity to ride it this weekend instead. It involves taking a few detours down dead ends but I'm looking forward to that - it's a good excuse to explore some interesting looking lanes that I would never ride down otherwise. Excellent coffee stop at halfway as well.
There's one annoying tile that's just a few metres off route - to get it, I would need to go at low tide and climb over some rocks...
These two clusters are all offroad so will need a separate visit on a different bike, or on foot...