In conclusion, thanks to everyone who responded, got a new battery fitted and all is back to normal.
The battery, believe it or not, is the original one that came with the car in 2005! I think it lasted well.
It's surprising how easily the pillar on the retaining arm sheers off. :-) Still, the other bolt seems to hold the battery in place firmly enough and the only way it's going to jump out of the cradle is if I roll the car, and if I do, the battery falling out of the cradle will be the least of my worries.