Yeah well I don't know why I didn't think of it earlier, shows how complete my conversion to tanks is....
I filled a Wow cart with NuMenthol & stuck it on the batteries in turn. All the auto titans except 2 fired the carto no problems. The offending two need charging before I try them again as I haven't been able to use them for a while.
So the batteries are fine, therefore it must be the cartos.
Update - the recharged batteries now fire the Wow carto. I replaced the cartos in both tanks, making sure I grabbed new ones from the bottom on my box to ensure I got a different box. Both read 1.6-1.7 on my VV. At least one of the batteries that weren't working on tank cartos is firing just fine at the moment.