Are you quite sure there weren't any roots going downwards...

Although tree roots are not thought to actually crack drains, they are
attracted to ones that are already cracked or leaking and pulling out
roots that have grown through a broken pipe may cause leaks...

Well I dug down nearly a foot all around the tree and cut all the roots
going outwards. Then I just pushed it (eventually!) and it went over so
there were no roots going straight down. Some of the roots may have gone out
then down. I found some of the roots just under the surface at least 10m
away (they went right under the surface of the lawn so they got pulled up)