Thought I'd update as we finally got around to getting his hair cut.
Went to W2K on Bedford Hill. Thomas, the hairdresser was very patient. Son was not. He wriggled, kicked screamed. Thomas tried giving him toys, pretended to cut my hair first, we promised him a treat if he was a good boy. Every trick in the book. Nothing worked.
Thomas suggested we go back when son was asleep, which we did. Thomas, bless his heart, crouched down and cut his hair whilst he slept in the pushchair. I had to take him out for the back of his head to be done, but he was too groggy to put up much of a fight. Hair got everywhere, all over him, me, the pushchair, but it was worth it to finally get it done. It's not the most perfect haircut I've ever seen, but that's in no way the fault of the hairdressers, they did a fantastic job dealing with tantrummy toddler.
And they only charged £7.50.
We'll definitely go back to them next time we have to suffer the ordeal again