Mike Clarke wrote:

Some manufacturers recommend that you do disconnect the battery. Some
cheapo chargers might have a very spiky voltage pattern that could upset
the onboard electronics.

Ah. I didn't do that...

Always connect the charger to the battery before switching on the mains...

....but I did do that, since it seemed the sensible sequence.

...and switch off before disconnecting to avoid sparks at the terminals.

And I shall do that. Thanks.