mk1 battery problem

[carspath]

i had the car radio on for about 15 minutes today with the engine turned off, and later the battery would not turn the starter motor to fire the engine
i then trickled charged it for about 1/2 an hour , and the car has started fine since then

the only other things that might be relevant are:

1)about 2 months ago, i tried to trickle charge the battery (not because of any problem, but only because i had not used the car for a month or so )through the cigarette lighter socket , and in doing so partially discharged the battery
i recharged it directly at the terminals and it has been working fine since then until the above mentioned episode

2) the battery is at least 7 years old

any advice /thoughts , other than the obvious (dont use radio without the engine running/new battery--an expense i dont want to incur unless genuinely necessary)
would be appreciated