sm_jamieson wrote:
I may have the opportunity to purchase some good condition used
double-glazed PCVu french doors. They are 5 years old. Will I have
problems with the building regs if I use them in a new extension ? What
if I can't prove what they are filled with etc.?

Difficult to say, but AFAIK the French make their
doors just like ours. Perhaps you could e-mail or
write to the manufacturer? Babelfish translations
might be useful if you don't speak the language.