Hi Kramer81,
If as you say it was a nearly new car, surely there must have been a dealers guarantee with it?
Without knowing the complete facts it is difficult to answer this question, but I would have thought that you would have some sort of claim against "Arnold Clark" especially as the leak only started 6 weeks after your mum bought it. Again the windscreen should be down to them, inform them that you would be prepared to go to the Small Claims Court if they dont pay up.
The petrol side of your story is down to "Experience" nothing you can do about it, unless whoever drove the car was caught speeding or whatever.
Regards