After making the two 1200 x 900 boards, it was decided to enlarge the layout and two more boards 900 x 400 were ordered. I knew the lead time for these boards was about six weeks when I ordered them, but I had little choice so I waited.
These boards turned up Tuesday and I made them straight away. Over the last two days I have added the new boards to the originals and each of the two boards now measures 1600 x 900. This makes the overall layout 3200 x 900.
The boards all went together really well. I doweled between the original boards and the extra boards to ensure they were very strong.