Dino,
A UK battery of the same spec will work fine; my back pack is a US E-bay one too.
And yes, I guess it could be a case of the battery running low; so just needing charging; but I guess you would've tried that first?
Also, you MUST fully charge a brand new battery for about 24 hours (to be safe), for it to function properly; I've read posts here from guys who've not done that and the battery doesn't last long (don't ask me why).
After heavy use, a battery will last you about 6 to 8 months; then will need replacing; so it's good to have a spare handy.
Also, charge it after every day you've used it a good amount. It takes seconds to take it out and plug it in; it's not heavy.
I've been using a backpack for three years now; and although finances aren't preventing me from going van-mounted; I like the way I/We work, so will stick to using a backpack. I don't want the extra expense of getting a van, or change of how we work.
I will add though that I prefer working with Wor Lass who trads the ground floor after I've WFP the 'tops', and in some places when I'm working alone, it can seem a real chore to then return to my starting point and start 'tradding' the ground floor.
Like most things though, there's a knack; and it'll take some time to get to know how to use it to best effect.