I got a strong positive at 12dpo on a Clearblue - this was about two days before AF was due. Also, when I got PG with my son 11 years ago, I used a Clearblue 4 days before AF was due and got a positive then too (not too strong but immediately visible anyway).
But, as Hevwithbump says, only half of pregnant women will get a positive result before the day their period is due.
If you don't mind the financial outlay (they are quite dear in comparison with other brands) then you could try again on Saturday, or some of the own brands are very good too such as Tesco's and Sainsbury's, and they're a bit cheaper (Tesco £4.95 for two tests, again I got a strong positive 2 days before AF was due). Good luck!