Sorry for coming in late but you can info on using OPK'S as HPT's on POAS.com, not sure what it says about doing it, I've never read it, just know the info is there!
And Sureit'sno.3 I can only see a line on the middle test, but that would have been enough for me!!
Cherub, a line is a line, as long as it has some colour, and it came up within the time limit, it is a line. And FWIW, test's that don't get darker doesn't mean impending MC or anything like that, different tests (even out of the same multipack) have slightly different sensitivities, your urine will sometimes be more concentrated than others, and really testing before your period is due you are only ever going to get very faint lines, some people will only start to get faint lines even 4, 5 or 6 days after their period is due! I didn't get one till I was 9 days late with DS2!!
I think the main thing you have to be prepared for with early testing is a CP, these are very common, but would normally go completely un-noticed.
Anyway, the pair of you, give it a few days (yeah right!!) then get a couple of FR HPT's, do them as you should with FMU, and get the right answer instead of all the 'maybe lines'