We got this little, v-reg Toyota hatchback for free almost a year ago. Been fine other than a puncture so replaced all 4 tyres.

Passed it's MOT last week.

Yet on Sat, the glw drove to see someone who then didn't answer. Sat there with lights on for a bit and went to leave. Car wouldn't start. Flat battery.

Spent ages trying, got a kid to help her pus it onto the side, and came home.

2 hrs later - so when it's colder outside and the engine would have cooled down even more - got a taxi with jump leads up there. But the car started on the key straight away.

Can anyone explain why in simple English and what I need to know so this doesn't happen again?