First Response, ASDA own brand or the Poundland ones are the most sensitive.
I got a positive with a FR one on a Wednesday, but then LOTS of negative tests until the following monday (inc using another FR one), so it was a bit odd.
But like you, I was certain I was, so I couldnt understand why all the tests were negative.
Anyway, on the Monday (when I did about 5 tests ), the first morning urine one was very very very faint, and I couldnt decide if it was an evap line or not. So 2 hrs later, I did another, an ASDA one, and the line was much much darker, so I dont actually agree with the first morning urine thing now
I'd go and by a First Response one, but thats just my impatience with these things!
Just make sure it's the one that says on the front of the box that you can test 4 days before period is due (or words to that effect), cos FR also do a normal preg test that isnt so sensitive.
Let us know how you get on. And good luck!