Thanks everyone for all your emails and comments on here and forums.
I have just received an email from Polyform Products Company
It made me smile to read the following reply which I have just received from Polyform Products Company (Premo, Sculpey etc)
Thank you for contacting Polyform Products Company and your interest in our products. I am so sorry that this project did not work for you. You are probably right in your assumption of why this did not work for you. Wood as a base for a clay project must be cured before baking. Because it holds moisture and will expand during the baking process (which is what happened with your project) it should be baked at 130°C for 30 minutes to bake out the moisture. After cooling you can proceed with the project as directed and it should be fine.
If there is anything else I can help you with, please contact me again.
What do you think of this reply?
Are you, like me, of the opinion that the least they could do was to say that they would edit their website project page. It just needs '1 x round paper mache box' added to the list of requirements - not difficult! And surely it's the most important part of the project and is not even listed!