Ivan, you are always welcome to post anytime, no matter how often.
It is easy for suppliers to offer advice on gapping and you can read as many books as you dare but I recon in reality for the absolute best gap is determined by a combination of the various texts out there and experience by decking installers.
Advice is usually best by TRADA but I do disagree on anything over 6mm, so there 10mm is a little excessive, even though for certain attributes of deck construction.
I do agree with your statement “But surely the most relevant factor is at what time of the year are the boards being laid, and are they kiln-dried timber, or seasoned, or unseasoned.” And of course your choice will be enhanced by using a moisture content metre. Again this detailed analysis is only useful on a surface fixed deck and not good what so ever on a side fixed deck as with the latter the gap is determined by the clip. Some clips allow movement for this to a certain extent but not all.
E.g., i’m specifying English larch from a saw mill less then 10 miles from the site in question. The boards will be reasonably freshly sawn, and it’s fair to assume, stored outside in the English autumn. So, it’s highly likely that when laid, the boards will be at their most swollen, and will likely contract in the summer. Presumably, a 4mm gap with wet, fresh sawn boards, could open up to as much as 8mm in the summer sun, and a kiln-dried board laid in August with a 4mm gap could possibly see those gaps close up entirely in a long wet winter. I’m using 145x32mm boards. Any thoughts?
May I ask why you have specified a timber not usually used for decking? One can’t be sure of its “best use” and I do know that this timber is rather soft – to the extent similar to pine and shouldn’t be confused with Siberian Larch which is more than double the hardness and completely different animal altogether. I would be certain this would be a lively product and I would recommend tight joist centres and also massive screws 5.5 x 75 mm (minimum)