Higgyboy:
If I had to work either the Tuesday in Wk1 or the Wednesday in Wk3 then would it become illegal?
No not necessarily. It may have a bearing on compensation for reduced weekly rest periods but based purely on the information given you could work either, or both, of those days and still be legal. If you were to work both of those days you would have a pattern of 6 day week, 5 day week, 6 day week, 5 day week etc. That is exactly my work pattern and it is legal.