When you are on the merchants list, use the 'amount tab' to sort the merchants out by highest amount so then you can see which are most worth doing. The best free money I've done has been for car and home insurance, credit cards (post office credit card 27.50, HSBC 25, egg 20 but space out applications so u don't mess up your credit file), there are magazine subscriptions where you can get a introductory subscription for £1 and quidco money of approx £6, you just have to remember to cancel within the intro period. Some of the bingo sites pay more than the amount you have to deposit, eg gala pay £20 quidco if you pay thru £10, so just use the £10 up then withdraw any winnings and you've made 10, some of the betting sites have similar offers but i've not tried them. love film pay 15.50 for the free trial (but again remember to cancel and only take out the cheapest 3.99 option so if u forget a month you've not lost much).
HTH!