Extended battery cases
It seems that these are not available in UK so far, except from US.
I have just received the Mugen Power 3000mAh case, and thought I should let people know.
Clearly the battery life has been an issue for some, both with the Nexus 4 and 5. I was in Canada with my Nexus 4, when the Zero Lemon 2250 mAh case with removable battery came out. While travelling I really needed a charge more than each night, so I bought it. Functionally it was fine, but really square, ugly and with a flimsy-feeling strip to fasten the phone in place. It had a built in internal microUSB plug to receive the phone, and a swappable battery. Then, it was about $80, and extra batteries were about $30! It is still working on my Nexus 4, which was passed on to my wife, although she doesn't use it often. Quite tricky to attach, but it did protect the phone to a certain tent.
I acquired a Nexus 5, and was quite disappointed that the battery lasted even less time than the 4.
This time I saw the Mugen Power battery first, US only but free postage. It looked slimmer and more stylish, and boasted 3000mAh. It was equally expensive at around $80, but being in Europe, free postage was a clincher.
I noticed after ordering that Zero lemon had trading brought out a new model for the Nexus 5. No removable battery this time, but reviews show a very protective case, again difficult to attach.
The Mugen Power case I received still almost trebles the thickness of the phone, but adds very little to the length and width.
Two negatives I hadn't anticipated. First, though the case can be attached easily and quickly, it offers no protection to the face and screen corners of the phone. Secondly there is no internal connection from the battery to the phone. To charge the phone you have pull out a fiddly micro USB connector which is on a short wire and plug it into the phone's own micro USB slot.
Both phone and extended battery can be charged at the with one charging lead when the short wired plug is used. The phone charges first, then the battery case. I wouldn't want to carry it with the connector in as it stands out and would catch on pockets etc.
In sum, if you need longer charge free periods, a battery case is useful, but the trade-offs are size and protection.