DOMINIE HEATING.
Problem solved.
On freezing high-level functional trips, by easing open the smoke removal "Ping-Pong ball" lever under the right hand pilot/student seat until just before the green DV Closed light illuminated, most of the discharge air then vented via the cockpit.
This ensured a comfortable temperature for the single pilot as long as the RPM didn't change too much. It was so effective that several time the SODCAT operatives in the back requested me to turn UP the heating!
All one needed to remember was to restore normal operation pre-descent, at which point the hissing noise acting as reminder would cease.
It always made me wonder why the smoke removal drill for the 'Dom' meant that any smoke in the cabin would flood the cockpit on its exit. And the DeHaviland 125 was a CIVILIAN design, approved by you know who, formerly of Kingsway in London.............
What WERE they thinking in the design office and the certifying authority??
Otherwise, great little aeroplane and fun job, pity Finningley was run by those of doubtful ability and intelligence......................