The Turing machine ran three instructions in a row on Wednesday morning. I caught the 9am Tuesday train down to London Hackspace and worked through till 9am on Wednesday. The only problem remaining is getting the ball bearings to land reliably on the correct grid space. Direction change and movement are all solved problems now. I've added a gate released by an existing cam to stop the ball bearings exiting the machine too quickly, but the ball bearings often miss their correct grid spots.
This can be fixed by placing guards onto the grid, but I really want to avoid modifying the grid. Modifying an infinite tape requires infinite effort.