Update: I used a 12v bulb to drag down cells 1, 2, and 4 whilst the pack was at full charge, and got everything within about 0.01v. So far, so good.
But putting on a 60A load, I've still got cell 3 about 0.1v below the others.
Does this point to a bad connection... or possibly a not so good...
My current thinking is to buy a pair of Victron Phoenix 1200VA and run them in parallel. But given my location its still going to be at least a week before I can get my hands on them.
The Epever warranty repair will be interesting, as I am now full time cruising and no longer in the country...