accident - who's at fault?

Hi guys

I parked my car on a snowy car park this morning. The car park is slopped towards the building I was going to and I realised it was quite slippy due to snow so I parked my car parallel to the building so I could easily drive away.

I entered the building and 2 minutes later heard a loud crash. To my horror I looked outside the main door and saw my car nestled nicely against the building!! My car had slid down the car park and has caused substantial damage to the exterior wall, making the main door unusable and jammed shut.

My car isn't in too bad shape but will need some repairs to the bumper and rear lights which I will probably pay for myself.

My question is, who is at fault? I stupidly do not have NCB protection so my insurance, which is due for renewal next week, will rocket from £25 a month to £175 if I they are to claim against me and I was to lose my NCB.

Any help greatly appreciated

Thanks