In my last post I erroneously used the OS grid square TD which was pointed out by BC who I am walking the OS Grid line 38 (SD to TA) with and some exchanges were made on our two posts.
In fact the square TD does not exist. Partly for refreshing my own memory and for others who may have had theirs similarly dimmed here is chart of all the OS squares. It will be seen that our walk goes through SD,SE and TA as I have indicated in red. At least it's less complicated than the Periodic Table. Somebody may make a fortune inventing a new board game based on this? Endless variations including endless combinations of squares to visit may be used for inventing new long walks - oh! don't get me started.