BeckiF - I'm not completely sure about this...but, those cd21 tests assume you have a 'normal' 28 day cycle when you ovulate on day 14 - hence they test a week later on the 21st. However, my cons told me to book my blood test in with the nurse for a week after I think I have ovulated (when my OPK says so!). So therefore, I think you're blood test would have def showed you haven't ov'd - because you hadn't by then. So in actual fact, your chart showed you ov'd on day 25 - you should have booked your blood test for day 32. The blood test looks for this hormone present AFTER you've ov'd.
I have irregular and long cycles and am on my first cycle of clomid. But I have had these blood tests for the last 4 months and I have got the corresponding results with my chart, i.e get a pos opk, go for blood test in 7 days, get a pos ov blood test. My consultant has explained this to me, its a wonder yours didn't. Instead I think you may have been given a false neg result on your blood because you just tested too early.
I hope this makes sense. Maybe others will disagree with this but this is what my cons has told me and this is they way I have been doing my (so called CD21) blood tests. They are called CD21 tests but they are based on that old myth that everyone has a 28 day cycle and so will then be ovulating on day 14.
So, this first cycle I ov'd on day 20 (OPK and chart showed this), I phoned the surgery and got appointment for them to take my blood the following friday (day 27). I'll ring the hosp in a couple of weeks to get results(or with a bit of luck i won't need to as I'll get a BFP before!). Hope this helps BeckiF. Talk to your cons about it. Let me know what the outcome is.
P.S If your chart says you ov'd and you did plenty of bd'ing, then there is always a good chance! If my above theory is correct! Good luck! 