I have an in car camera, takes it,s power via a usb lead and ciggy lighter it,s pretty good and i would like to use it
as an action cam with a battery on a lead kept in my pocket.
having a look at fleabay i seem to have 2 options, a sealed lead acid battery (house alarm), sounds heavy and maybe dangerous or a
battery holder with 4 AA batteries in it, that would be my fave option but.....how long would 4 AA batteries last ?

just get a usb 'powerbank' , poundland do some, and you can go up in capacity (and price of course) from there.

a selection from a well known retailer:
http://www.argos.co.uk/browse/technology/mobile-phones-and-accessories/mobile-phone-accessories-and-attachments/portable-power-banks/c:30297/

good idea, thanks ill pop down there morra

I'd suggest a USB power pack too. Rechargable, I've seen them up to 30,000mAH and most seem to offer a 5V DC / 2.1A output (many smartphones like this for charging these days). If you don't see what you like in Poundland then ebay and others sell them as well.