For some time I've been having a misbehaving speedo, that wobbles about, abruptly hits zero and stubbornly remains there etc.
Anyway, last week, the car wouldn't start and I had to be recovered by the AA. The mechanic found that the battery wasn't charging.
The garage spent three hours looking for the problem and eventually diagnosed two:
The speedo had a faulty cable.
The cable from the alternator to the battery had corroded, causing it to create resistance and therefore preventing the battery from charging.
Total: £150 quid!
However, I took out a warranty policy with the AA whereby if you have to be recovered, they will pay for any repairs required to put the car back on the road (except for wear-and-tear items) up to £500 with £25 excess.
This costs a fiver a month and you can claim up to three times a year. The upshot of it was they paid for the alternator cable plus diagnosis (labour) - £98. So it's already paid for itself. Bargain of the century!