Sleeping Bag Recommendations

I want to buy a sleeping bag for winter camping. I have tried googling but there are so many types and brands I'm confused as amazon reviews seem hit & miss. I was hoping someone may have some advice

I would like a sleeping bag that is lightweight as I will be using public transport but warm enough to cope with the upcoming cold weather so one that goes into the minus figures. Thank you for your help, I really am clueless when it comes to this.