In article ,
Martin Brown wrote:

Your least worst option now is to charge it for a couple of hours with a
dumb as a rock simple lead acid battery charger to see if you can get
any charge to stick (ie. enough to start the mower). If it won't hold
charge for at least a week you are stuck with replacing it.

Ideally an initial charge rate of C/10 to C/5 but keep an eye on it.

Smart chargers will sense battery parameters and do a safety cutout if
the battery has gone high impedance or has one cell reverse polarised.
The lifetime of the SLA has been compromised - possibly to zero.

Thanks Martin ... yerrrsss: my charger is "dumb as a rock": just has
lights (no dials or meters), and very little in the way of controls (in
fact just one switch: 6V or 12V?!)

I regretted buying it as soon as I got it, but it cost about 35 quid and
was reluctant to spend more, especially as I myself know almost nothing
about electrics/electronics.

I'll see what happens next time I go across and give it another charge.
I'll report back, as I really do appreciate the help you've all supplied.

John