Power pack is ok if you are only going to use it for smaller items, just see it as a 20amp/h battery, spend a bit more and get a battery and panel to charge it.
Power pack solution
Thanks all. Joiner is knocking up a box to fit a 12vmotor bike battery which is half the weight and size of a car battery( and capacity ) Will fit a cigarette lighter socket in the box. This gives me exactly what in need, good quality and contributes a little less additional junk to the world. Thanks all for you advice.