Finally got around to updating the "story so far". Just before Easter I put together some equipment.
I took this lot back to the site and was very pleased to discover that the whole lot including my tool box could be lifted up to the top of the scaffolding with a fork lift crane. I put the box in position and put the board over the holes in the chimney. A piece of stone and a lightweight concrete block held it in place.
The other day I went back and with the help of one of the carpenters we have repositioned the board and sealed around the outside with expanded foam.