I've got a starting problem with my 2000 2.2 CD auto

the car can be left for days on end and it will start no problem but if I've got to wait in the car for the misses,which is quite often, with just the radio on and the ignition off for about 15-20 mins the battery is flat and wont start the car. when i first bought the car i could use the radio for hours on end and it would still start.
i think its the battery as its a vauxhall one so it may be the original

but has a anyone any other ideas