TopCashBack and Quidco have been good, although the opportunities for regular easy money qithout making a purchase have all but gone. They are trustworthy but some merchants are extremely slow at paying the cashback- sometimes you wait weeks, sometimes months, sometimes 12 months- and some never pay up
There are payment speed statistics but I take them with a pinch of salt. Quidco used to have forums where members discussed slow payers/ those who never paid but that seems to have gone. Have you got a particular merchant in mind as I may have experience