Obselete now, but think this is the one I have
https://www.amazon.co.uk/Advanced-DeWalt-DC820-Impact-Wrenches/dp/B00I9F7FOWI run it off 2Ahr nicads IIRC. NiCads have the ability to give more punch than Lithiums, at the expense of running time, but if I was buying a new set now, I think I'd strongly consider Lithium tech.
Given the decent prices (they are always on a good offer somewhere), and my experiences with the rest of my matching set (XRP drill, right angle drill, DC825 impact screwdriver, leccy screwdriver, impact gun, torch etc), Posh wank&decker (DeWalt) would be near the top of my list. Though they do some shite as well, like the plastic chucked drills that shitfix pile high and sell cheap, which aren't a patch on the real stuff.