Collapsing loaf baked in the oven

I've been making bread by hand for 40 years but have never solved the problem of my loaf top collapsing during baking. I use total 1kg flour: 70% whole meal and 30% strong white and use Tesco's dried yeast sachets. I mix by hand, prove in a warm oven, knock down and do second rise in three  tins. 

I normally use 1 sachet for 1kg flour which is half what it says on the yeast packet. But I've also tried using 2 sachets. I've tried more water, less water, conventional oven, fan oven, hotter oven, cooler oven. Preheat oven, don't preheat oven. Single rise in the tins. I've tried three different makes of flour. I even tried fresh yeast.  Nothing has made any difference! What could I be doing wrong?