I would definitely look to replace your mums, the rubber can degrade over time. The little flower pattern on the neck of the bottle is there to let you know how old they are, but if they're older then you, they're definitely out of date.
I just prefer the feel and heat capacity of hot water bottle's to the weat bags, which as others have said have their own safety issues anyway. I don't mind an electric blanket, but have kids that often turn up in my bed over night, and I don't trust 100% not to wet the bed, so not a great mix with electric. HWB just gets kicked out the bottom of the bed if/when I have a night time visitor.