Thanks for your interest. No, the battery wasn't returned, but having mentioned it registered 6V the supplier and warranty firm immediately said that a reading lower than 10.6V voids the warranty.
Understandable that they might set such a condition, but a battery below 10.6v could be due to it either being faulty, or abused.
I'm not sure what "magic" charger a garage might use but mine is fairly sophisticated, it charges various battery types and the voltage and amp settings can be changed - I've tried various combinations.
One of the problems with modern, more sophisticated chargers, is that the will not even attempt to begin charging a deeply discharged battery, if they have dropped below maybe 10 or 11v. The workaround, is to temporarily connect a second good battery, in parallel with both battery and charger for a while - long enough to get some voltage into the battery.