Actually, guys - I'm a thick Geordie git (as many of you know) - and because I expected the light to go red and stay red until the battery was charged, I assumed because the light was green, that it wasn't charging.
So anyway, I took the backpack outside, filled it with water and turned it on. Straight away the battery indicator 'jumped' to green and the backpack emptied it's contents.
I then put the backpack onto charge, and the light went straight to red (as I guessed it should since I've just used it), and it's all working great. Maybe the battery was fully charged when I received the backpack?
Anyway, it looks like - for the money - a good bit of kit. I just wanted it as back up for my shurflo backpack; which gets a right hammering and is still going strong after about three years!
Thanks for your replies, some of which are still useful, and apologies for bothering you.