I've tried this and it works! It has now been validated at Quidco. You MUST use the link provided - signing up on their regular site does not give you the 4 weeks free. Go into your Quidco first, then Tesco Diets, then C & P the web address.
12 weeks @ £2.99 per week = £35.88
4 weeks free, via the link = £23.90
£15 back via Quidco = £8.90 for 12 weeks
£8.90 divided by 12 weeks = 74p per week!