Well, I've found out what drained the battery the other day I think.
I fell asleep on the sofa last night, woke at about 02:30. And I thought I'd stick my head through the curtains to have a nosey before retiring for the night.
I looked out and I could see red reflections in the windows of the houses opposite.
I went outside and found my brake lights were on!!!
Opened the car, and touched the brake pedal with my foot and they went off...
Think it needs a new brake light switch?