Richard Bertinet bread

Hi, looking for some advice about strange bread texture! I have recently signed up to Richard Bertinet’s online course and I’m trying the basic recipe- 500g bread flour, 350ml water, 1 sachet yeast, 10g salt. (I’ve also tried once with 7.5g salt, and once adding 1tsp sugar) 

Following  his method my dough looks lovely, it’s really smooth and silky and light. I’ve tried by hand and in the mixer and looks good both times. The first prove always takes longer than he says because it’s quite cold in my house, so I often use my fan oven on a very low heat and cover the bowl with clingfilm and a tea towel to keep it moist and this improves the rise. Then I shape it with his method which I am struggling with a little, and the second prove is again quite slow. 

My first two loaves were not good, they had a dense but holey texture almost like crumpets. My most recent loaf seemed to rise well, and I baked

it for  35 mins (10mins at 230, 25 at 210) and it did rise in the oven and looked really good but inside the bread still seems a bit dense and gummy at the bottom and goes hard pretty fast- maybe this time it was underbaking, but I don’t know what happened the previous times? I’m not sure what I’m doing wrong, I’ve made bread lots before (quite often using the oven as a warm place for rising) and never seen this!