I have (elsewhere) previous been recommended to buy a power bank to uses as
an emergency back up for my portable devices.

Which, because I have a candy bar phone with a standby time of 3 weeks,
means my tablet and camera.

I was in a shop today, saw one on sale with "for smartphones and tablets" at
a reasonable price and bought it

but after I looked at the back it said output 5V 1A.

Oh!

ISTR that when I bought my in-car USB charger I got one for phones and had
to take it back and get a higher rated one for Tablets. And even then, one
of my tabs reports that it isn't powerful enough to "charge" the device,
though experimentation shows that it does keep it operating for a bit
longer. Plugging my camera in and it doesn't even notice that there's a
charge there.

So I went into all the other shops in the high street to see what they had
(lest I should want to take the purchase back whist I was still there) and
all of them, with one single exception were output 5V 1A, that exception was
fugging expensive and more importantly 3 times the dimensions and 10 times
the weight of the one that I had bought. I particularly wanted a small,
lightweight, fits in your pocket example.

Back home I looked at my plug in the wall USB chargers, which I
(successfully) use interchangeably on all my devices and one says output 5V
2A and the other output 5V 1A.

What's going on here?

what rating do I need to charge my device(s)

I did a quick Google to see if I could find out and found "5 Key Things To
Know When Buying A Power Bank" and one of them is "When you buy a portable
power bank, make sure that it can charge the battery of a specific device"
Yeah, I know that? But how do I find out what that requirement is - I came
here expecting that you were going to tell me as the manufactures of these
devices keep it as secret as the coca cola recipe (I have been online and
downloaded the full specs).

So the item in question is still in its unopened box with all the seals
intact (as it's from one of these shops that aren't the best for taking
stuff back to). do I take it back and seek out a more powerful one, or open
it and try it, and risk the shop refusing my returning it?

tim