Thanks for the link Paul, that must have been rather frustrating going back and forth like that over a number of months.
The test roll, with a different 120 back, was posted yesterday so I'll hopefully have it back during the latter part of the week to check the results (it was also pretty cold yesterday AM so it should be similar conditions to last time).
I just hope the lens arrives in the next week or two to do some more tests, if it does appear to be the lens then I'll have to check if they'll do a partial refund or replacement depending on their stock.
Interested to try that silver oxide battery as well just to rule most things out.