In article ,
"www.GymRatZ.co.uk" writes:
On 07/02/2012 12:52, Roger Mills wrote:

I think that what I actually want is a manual stat with an operating
range of a few degrees either side of zero, and with an external sensor.
[The stat itself doesn't need to be weather-proof as long as the remote
sensor is.]

Any idea where I might find such a beast?

Straggling OT once again....
Why not dose the system up with an anti-freeze specific inhibitor.

Expense?
Andy Hall had an outdoor workshop which he heated from the boiler
in the house via a plate exchanger to a separate outdoor circuit.
The outdoor circuit was dosed with anti-freeze. It was sufficiently
expensive that he designed in the ability to drain the circuit into
a collection tank if any maintenance was required, and to refill
from that tank, rather than to lose the anti-freeze.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]