As a commercial user I believe (or rather have been told, but not seen the evidence) that we are charged for neutral imbalance.
I think you've been misinformed. The DNO doesn't care about balancing of the phases .... a zero neutral current is pretty much impossible in an installation that contains anything other than just motors anyway.
On a commercial tariff the DNO will be concerned about power factor & you may well be penalised for having a poor power factor but that's not the same thing as neutral current.