Thanks for the quick response guys. The temperature has been quite low here - the 147 did register -10 at one point and the car is still on its original battery. It was my wife who experienced the power steering thing but I think she may be talking pants!
I tested the alternator and all is fine there so hopefully it's just the battery.
I'll update the post tomorrow and let you all know how I get on. Hopefully a battery and new crank sensor next week will solve the problems but I'll update as things progress!