Sounds like you have a big mouse infestation, but you will be able to get on top of them with trapping. You really need to find out how they got/are getting in to the loft though. I personally have had greatest success with the "Little Nipper" traps, I have the one you used there, which I believe was from B&Q but the little nipper is the best, albeit a little 'messy'(!). Avoid ones from the "Big Cheese Company" which in my experience are very poor build quality. What are you using for bait, I would be running quite a few traps at the same time, how many have you got at the moment?. With half a dozen (and ideally more) traps you will soon thin them out, may be worth trying a few different baits on them, cheese, chocolate and sunflower seeds or peanut butter are three of the favourites.. Poison will kill them (they are neophobic (scared of new things) and so may take some time to eat it), but with poison you won't be able to dispose of the bodies and the smell is horrific.
p.s. you can buy 5x little nippers for £5 (including delivery) here: http
/cgi.ebay.co.uk/X-5-LITTLE-NIPPER-MOUSE-TRAPS-NEW-/120523784858?pt=UK_Home_Garden_Garden_Plants_Weed_Pest_Control_CV&hash=item1c0fc7029a