We have finished investigating the problem with our under-floor ducting and why it was not blowing through with sufficient volume of air. It seems to be clear in the channel between the two layers of PU foam (not completely sure without resorting to a fibre optic camera!) but it was noticed that there were many leaks along the concrete floor and the side walls of the channel.
We think that much of the air is leaking out on both sides of the channel but we cannot reach them and put in a caulk filler, except for a short segment where we can lift the floor cover up from over the Earth Tubes. It was quite noticeable that even just that small segment after it was filled with the caulk filler, the increase amount of air flow. But alas not enough for our temporary living quarters!
We have learnt a valuable lesson with air ducting! Seal It Up!! Do not assume that a large wooden joint screwed together is a perfect seal!
The state of this ducting is just about ok for the Garden Room and Garage when it comes solely into its own. But for now we will have to put in another temporary ducting to deliver fresh air to our temporary living quarters.