George wrote:

I would like to fit a small extractor fan into the double glazing. I
smoke in the conservatory and not having to have a window open in the
Winter would make life a lot more comfortable for me.

Is it possible/practical to fit an extractor fan into conservatory double-
glazing ?

Yes

Any advice or assistance would be gratefully received.

There's a section in the FAQ about fitting a catflap into double
glazing, which should be helpful:
http://www.diyfaq.org.uk/miscellaneous.html#catflap

Alternatively, why not take this as an opportunity to give up smoking? :-)

David