I'm quite certain you're entitled to it as anybody working more than 30 hours on a low wage can claim Working Tax Credit
You are right that WTC's are means-tested and there is a cut-off point, but I'm not sure the wage you can be earning before you can no longer claim. Suffice to say your annual entitlement must be less than £26 before you can no longer claim. therefore I'm 99% sure the cap is way more than £12,000pa.
I might able to work out what you're entitled to if you can tell me three things. Have you been on the same job and wage since April 2010, are you over 50 and do you claim DLA for any disability which makes it difficult for you to find work (even though you're working now)? This won't affect your ability to claim when you're already working 30+ hours, but rather how much you can claim.
If you don't want to disclose any of that information online, then visit your nearest Citizens Advice Bureau. Don't bother with JobCentre Plus, they are often unhelpful, plus it's HMRC that is reponsible for Tax Credit payments, not the DWP. There is a test on the HMRC website as others have mentioned, but it's very easy to get an inaccurate answer.