Prof. Evans is a lovely, genuine and knowledgable bloke but it is worth remembering that he has a very specific take on the 'purpose' of woodland and our relationship with it. His perspective is one that is informed by his christianity - that is that we are stewards of God's creation and that there is almost a duty to work with God's creation to the end of useful productivity i.e. timber.
It certainly isn't the only approach. Different situations, woodland histories and potential wildlife, all need to be given thought before deciding to follow thinning strategies that favour the rapid growth of trees that are likely to produce 'good' timber for the market as the ultimate goal.
A "diseased" tree is one playing host to fungi or invertebrates different to those associated with a "healthy" tree. It's all a question of perspective. In deciding if you are going to thin with the aim of producing "good" straight timber - i.e. trees without nooks and crannies, forks, rot holes, dead branches then really consider if a small wood is ever really likely to be a good bet financially for timber? Perhaps a small wood with trees that cause offence to a forester, but with greater biodiversity are really what you might have been aiming for when you started playing this game...
I'm not criticising Julian - just wondering if his approach should neccesarily be thought the 'correct' way to manage small woods?