kytelly wrote:
I'm trying to use the journey planner to work out the best walking
routes around London.

However for anything over 1 hour it sometimes claims no route
available, but this is intermittent. I've tried ticking the various
walking only options but to no effect.

Any tips for success here?

It used to be unlimited, and you could get a walking route from Romford to
Uxbridge if you wished. I presume they introduced the limit because they
realised people were using it to calculate driving routes.