I thought to update the above - in case it helps others.
I spoke to my lawyer - he said whilst he understood the request by the land owner, it was more of a requirement for a business or organisation such as the Scouts. For a member of the public, the request was disproportional to the risk I posed. The main risk of a member of the public being on private land is a claim for financial loss for injury. My lawyer's advice was to phone the land owner, explain the above and offer to write a letter confirming I would not seek to claim any losses.
I phoned the land owner. He understood the points I raised and agreed, the provisions he requested were more suited to an organisation and therefore his risk/concern was around that public liability. As I was a member of the public, he was happy with an email response saying that I would not seek losses for injury and that I would not seek to cause damage. He also requested he knows when I will be on his land. He was supportive of people being in the wild and learning about nature. He will provide a permission email, so if I get challenged about wild camping on his land, I can show I have the right to be there.
Final thoughts - at first this seemed very daunting and I was worried about the conflict. I believe the land owner was pleased I contacted him, as it meant he knew who I was and why I wanted to be on his land. I believe he was pleasantly surprised I contacted him as other people just went on the land without permission. I believe it helped that I we are a father and son combo, he related to wanting to be outdoors and in nature, and that I was polite, courteous and respectful.
Good luck to anyone else in a similar position.