Found the answer and a workaround which may help others so here they are: -

1. Word uses .dotm template and cannot be changed. The .dotx template was last used as default years ago but they can still be created and used. It is just not default any longer.

2. Workaround - I have created a shortcut on my desk top that points to the .dotx template which opens with the customised page colour.

Sorted.