I was reading comments about oil or no oil. I use butter, but when cooking a 50/50 white/wholemeal loaf it is so soft its like cotton wool and nearly impossible to cut. Any ideas how get the loaf a bit firmer
For a firm loaf you want more gluten development. So:
Cut out the fat: it impedes the reactions forming gluten.
Make sure your yeast or sour-dough starter is active, or use instant yeast (very reliable in my experience): the carbon dioxide created as a by-product of fermentation promotes gluten development.
Use stronger (that is, higher protein) white flour rather than AP flour
Work the dough thoroughly: it is hard to overknead by hand. Several short kneads can be more effective than one long knead.
Try giving the dough a long rest, for instance overnight in a fridge.
Avoid granary flours or seeds with sharp edges: these cut the developing gluten network and make for a weaker loaf.