Gaffar wrote:
I am having an extension built with the work likely to start bang on in
the middle of winter.

Silly comment?

Do it inside.
Call a few marquee hire companies.
Alternatively, you can knock up a weatherproof structure out of 2*6 and
galvanised sheet pretty fast, then work under it.
Or, even cheaper, tarpaulins held off the structure by a minimal
framework.