I had an early bath, then made us a breakfast/brunch, to see us through to a late dinner.
An hour ago, the replacement pair of 12v batteries arrived, to replace the ones which I discovered had died over the weekend, on the stair lift. As the chair can sit unused for months, long ago I decided it would be a good idea to switch the charger off and had a switch to isolate the pair of 12v batteries (two in series, 24v), just switch it on as it might be needed - avoid wasting energy, and the improve the life of the batteries. What I had noticed, was that there was a centre tapping for 12v to something, so switching off, still left one battery supplying a load.
Before adding the switch, I had asked the service engineer, if there was some way to be able to turn the charger off, and isolate the battery. He had suggested the switch on the arm would isolate the batteries completely, except no it didn't and that wrecked a previous set of batteries.
Delving more deeply into it, I found what seemed to be a master switch hidden at the back of the chair, which does indeed isolate the batteries completely. Strangely, there are no less than three switches on the stair lift - a key switch, an on/off switch both on the arm, then this newly discovered master switch at the back. Turning the latter off, and checking for current draw from the batteries, there is none - so that switch at least does seem to isolate the batteries completely.