I think the gap at the post will attract moisture with or with out the v-groove. The inside of the post will be warm and the outside of the post will be cold, allowing repeated condensation when the conditions are ripe. Maybe it is of no concern?

I don't know what the minimal amount of insulation needed on the outside of the post is to stop this from happening.

I keep going to clay and straw or wood chip as an answer, with the post partially buried in the wall of clay/wood chip, say half in and half out, half exposed to the inside of the building and completely covered on the outside, with plaster and render applied.

I am starting to get frustrated with all the options, this is what you get if you want a variable, keep looking.

All the regulations coming down the road is a headache. I will just stay home and work on my gardens and stuff. That is probably what They want anyway.

Define economical?

Not everyone will go for a timbered frame either. Why limit yourself? Where do you draw the line? Rhetorical questions.

Tim