UK Ladies - where do you buy your opk tests? Which brand do you recommend?

I've been using opks for about a year and have never really been satisfied with the results. I'm never 100% sure that the test line is as dark or darker than the control line. In Novemeber a friend gave me a box of Clearblue advanced digital tests and for the first month I loved them. I had a low day, followed by four high days followed by a peak day. In December I tested about 10 days in a row with one low day followed by highs - no peak. I then dropped the test down the toilet so couldn't test anymore that month!! I bought a new test and tried again in January and again had a low day followed by loads of high days. This time I kept testing up until AF - high the whole time!! I'm starting to think it's a ploy by Clearblue to get you to use as many sticks as possible each month! So I was wondering what opks you use? Where you get them from and whether or not you definitely get a test line as dark or darker than the control?

Thanks in advance.