I have used two Halford Powerpacks for almost eight years and they have been shipped to Australia and back. They still work fine (I charge them fully after every usage though). I am only now considering changing the batteries in them in the next few months.
I would get the larger powerpack with at least 17Amp hours. I was in Halfords this afternoon and see they are selling a 33 amp hour pack (pricey at €199).
The newer packs with usb only have one 12V outlet so you will need a splitter box if you plan to use dew heaters with it.
Alternatively you can get a 12V leisure deep cycle battery and make your own box for it. It will have far greater capacity and last a very long time if charged properly. I think there are some plastic battery boxes available with 12V sockets you can buy for them.
The Lord giveth, the Revenue taketh away. (John 1:16)
