BATTERY PROBLEMS!!

hi all my battery keeps going flat every couple of days and i dont know why???

i have:

checked alternator charge level that fine
put a brand new diesel battery on the car
checked stereo

i have a laserline alarm on the car witch has 2 flashing led's and the alarm works the central locking and fuel pump etc etc

i dont know whats causing it to keep going flat and was hoping someone could help please


the car is a:

FORD SIERRA SAPPHIRE
GLSi
1992 (k)
2.0 DOHC
PETROL


thanks