Half Marathon Heart Rate - what zone do you race in?

You don't.  You leave the HRM at home.

(But ..... for the record you should be running a half right at or a beat or two below your LT in most cases).

I personally race somewhere in Z3, with the last 30 minutes or so in Z4.  The last 1/4 mile in Z5.

I have a hard time leaving the hRm/garmin at home, I like the data :-)
So I guess you are saying slightly slower than 10k pace???? 10 sec's per mile slower???

Depends.  The slower someone is, the more time between their 10k and half paces.

If you have a current race time you can input it into the McMillan calculator to get an idea of what you "could" do if properly trained:

http://www.mcmillanrunning.com/mcmillanrunningcalculator.htm

Your half pace is right at what your tempo pace would be (they are one and the same by most coaches definition).

But racing HR is not the same as training HR and so your zones would be a little bit skewed and more than likely racing by your training zones will leave you short changed if you are looking for the optimum result.  Which is why I said leave it at home (or completely cover it up if you absolutely have to have the data for later).