I had the same experience growing cauliflowers in our large tunnel this year. I planted each plant in a square foot 'square' (based on the square foot method of Mel Bartholomew) , enriched with 4 handfuls of garden compost, 2 of weathered wood ashes and 4 of vermiculite (helps drainage and keep moisture in our clay soil). I also gave each square a handful of organic fertiliser around the plant. We watered regularly.
I had been extremely sceptical that we would get anything more than tiny hearts at this spacing, but of a dozen plants we had at least six very large caulis, 4 moderate size ones, one small, and one which succumbed to cabbage root fly. Like you, I began to think that they would be all leaf, but they hearted up perfectly well. Depending on when you planted them, I would guess they would start to heart up soon.
As long as plants are large and healthy they will always produce a reasonable heart; it's the plants which are weak growing and have dried out which produce only a tiny heart.
I must admit in general I find the polytunnel is better for brassicas in spring and autumn than for traditional tomatoes, peppers etc. You can get several crops a year and protect things well into winter.
Good luck with your savoys and caulis. I'm sure they will impress you.

Thank you so much, I am encouraged by your comment !! I do look
forward to my crop.