Don't insulate under the floor of the loft extension, unless you want it to be a cold room for hanging game or suchlike.
Starting at the outside, tuck your glass fibre down the cavity , say 3", lay it across the ceiling, up the walls of the extension and across the top of it down the other side, finally ending up in the cavity again. the idea is to enclose the habitablle rooms within a tea cosy, the soffits are outside of the house, as are the outer skins of your cavity wall. the pupose of tucking ther end of the insulation down the cavity is to slow the airflow down, else any unnsleeved airvent in a wall will turn your inner cavity wall innto a very cold one!
I would not use expanding foam for any purpose where it is contact with plasterboard. The reason is that the final volume of a squirt of foam is unknown, it expands for hours (>12?) after the first squirt, while its expanding it can exert great pressure. I have seen a cavity filled with foam, lift a window board 6' long which had 3 of 18" hollow building blocks on it (30lbs weight?). it would bow plasterboard without any difficulty.
I would not get too precious about the mould, when the loft dries out it will die, and unless you intend to spend a significant portion of your life, footling about in the loft the risk is minimal.
Frank

Or alternatively you could consider mould as an avoidable health risk.

Said on the tin of foam filler not to be used under 10C so I guess the snow saved my plasterboard ceiling !

Thanks for the tea cosy analogy and description Chuckie, that's helped me understand how much new insulating is going to be needed to get things sorted. The big problem is going to be accessing these tiny spaces! Might be I have to get the soffits removed so I can get into some of these spots.

Apart from the first floor being slightly (!!!) on the cold side and an increase in heating bills what are the short term risks of leaving things as they are until the spring or summer? In an ideal world i'd love to get someone to do a good job on the roof but I don't have the spare cash at the minute and now is not the time to be taking out a loan with job security being what it is at the moment.

I've been quoted approx £6500 to have the roof taken off, joists and floor ends insulated correctly, a new felt membrane installed and the original roof tiles put back in place.