UK Ancestry - delayed arrival

I have a UK Ancestry visa which is valid for five years starting 21 Nov 2013. I put off arriving in the UK because a truly unmissable job came along, which was short-term enough it wouldn't make much of a dent in my time there, and would open major doors for me when looking for employment upon arrival. This project is now going a couple months over schedule, which leads me to a certain vagueness in the visa rules. The UK Border Agency's website states:

    During the continuous residence period, you cannot be outside the UK for more than 180 days in any consecutive 12 months. Absences must be for a reason that relates to the reason for your stay in the UK, or for a serious or compelling reason such as serious illness.

The project probably has me staying here more than 180 days past my visa's start date, which might put me in the danger zone re: The Rules, so I have some serious questions:
1.  Does the 'continuous residence period' start from the active date on the visa, or from my entering the country?
2.  My absence is because of work, which is the same as the reason for my stay, so does that affect things at all?
3.  Is an extended contract sufficiently 'compelling'?
4.  I have the opportunity to go over for three weeks in February/March; would this be advisable? (I'd like to know well enough in advance that I'm not buying a last-minute trans-Atlantic plane ticket, but y'know, needs must.)

I've asked these questions of WorldBridge, and the Foreign & Commonwealth Office, to which they forwarded me, but have got no answer.  The nearest British Consulate will only talk to British nationals in distress.  I put these questions out on my blog and got some helpful feedback (not least of which was a lead to this forum!) which suggests the 180 days start counting from the day I arrive, but I'd like to get this information corroborated if at all possible because it will affect how my next couple of months play out.  Ideally, I'd love to be pointed in the direction of some official statement of policy in this matter, so if I get hassled on arrival I can have something more solid to give them than 'I heard it on the internet' — but aside from that, if anyone here has personal experience with a situation like this or knowledge of current visa policy, I'd be very grateful for any feedback you're willing to give.

Thanks for your time!

1.  Does the 'continuous residence period' start from the active date on the visa, or from my entering the country?

It starts on the date you enter the country. So, your residency period does not start until you move to the UK.

To meet the residency period you have to be physically in the UK for the full 5 years, so if you have delayed your entry to the UK by several months, you will have to extend your visa when it expires instead of being able to apply for permanent residence right away... as you can't apply for permanent residence until you have been in the UK for the whole 5 years.

2.  My absence is because of work, which is the same as the reason for my stay, so does that affect things at all?

No, because you aren't 'absent' - you haven't even entered the UK yet, so your residency period hasn't started.

3.  Is an extended contract sufficiently 'compelling'?

Well, as you aren't in the UK yet, you have not been absent for any reason so far, and so it is not relevant.

4.  I have the opportunity to go over for three weeks in February/March; would this be advisable? (I'd like to know well enough in advance that I'm not buying a last-minute trans-Atlantic plane ticket, but y'know, needs must.)

It depends - it will start your residency period, so after you leave the UK in March, will you be able to return to live in the UK within 180 days in order to meet the continuous residency requirement?

If you enter the UK in Feb/March and then will be absent for more than 180 days, that's when all your questions will become relevant.

I've asked these questions of WorldBridge, and the Foreign & Commonwealth Office, to which they forwarded me, but have got no answer.  The nearest British Consulate will only talk to British nationals in distress. 

Yeah, Worldbridge won't be able to help you (they are not immigration advisors and only have access to what is written on the UKBA website) and you cannot contact a British Consulate to ask.

Best thing to do is to look on the UKBA website and ask on a forum such as this one.

Thank you so very much for all this information!  Such a relief to know!