Randometer said:

Maybe my multimeter is phucked? But that was one day after the charging!

Click to expand...

I would think that is too much for a good 12v battery 24 hours after charging.

But the original 12.3 volts you posted about is a bit too low.

Anyway, run with it, are you sure that both ends of the battery leads are making a good connection?

Thanks for all your replies folks.
I've just checked the battery again and...
It's back to 12.36 volts, so with what you have said and my own feeling of it sounded a bit less vigorous on turn-over, I'll be getting a new battery.
Hopefully that will fix the problem.