2600mAh USB Power Bank Portable External Pack

Well, it'll still cost you £3.99 - 99p is just for the cable :)

It should work with Espruino - but:

• Sometimes Espruino doesn't draw enough power to convince these things there is anything connected. You may have to add a resistor to increase the power draw.
  
• Because they have a 3.7 (Li-Ion voltage) to 5v converter inside (which isn't that efficient) you may not get the battery life you're expecting.
  
  

Of course you could crack it open and fit a cable directly from the battery to Espruino, and then you should be able to run Espruino on it for ages.