Peter as explained privately the steps you have taken are not a fix, just a sweep it under the carpet.
As a temporary fix until you get a replacement battery, it is fine, as a proper solution you need to change the battery.
The current battery is not able to sustain the panel properly in the event of a power cut from what you have said.
I suspect you will find out at the next power cut.
Fair enough. If I do find out at the next power cut then I'll change the battery. Till then, I'll leave it well alone.
But to clarify, the battery appeared to power the siren pretty loudly!