Got the alarm apart and found this...

[Green Goblin]

Hmmm...

Decided to give the batts a quick measure as the alarm is still on my living room table while the brakes are being attended to at the garage.

Both are now showing 3.70 and 3.71 volts respectively, after just a couple of days sitting. So I'm thinking that (as often happens with car batteries) the voltage reading by itself is simply not good enough to determine the condition of them. I'm betting that they are probably on the way out in terms of their ability to hold a charge. And, actually, the high reading (4.1v) may even be an indicator that they're defective.

Whatever their condition, I'm gonna change 'em. Be rude not to while the alarm is out of the car and I suppose I'm one of the lucky ones in that I'm doing this before they've leaked!

Any recommendations to source Varta Ni-MH 3.6v 140mAh with the 2 pins either side of each battery? Most of the ones I've seen have the 2+1 pin configuration, which wouldn't be the end of the world but they'd need to be modded slightly and mounted end-on (i.e. tall not low) so as to fit.