Phil
Your car has GEMS ignition/fuelling? A common (no very common) cause of the battery flattening is the back voltage through the ignition after switch off (sorry I am no auto electrician, so they will wince at this explanation)
The way to counter this is to fit a warning light into the circuit which warns you that this has happened , and when it does you cycle the ignition on and off again until the light extinguishes.
My car is Hotwire so I have not had to do this, but I ma sure others like Spanner Juggler or Rovert will be along soon to advise exactly where to place the additional wiring to feed the warning light.
