Thanks for the pointer. I was using the default style from the template, which had both model and layout scaling ticked. I had previously overridden the text size in the dimensions rather than altering the styling. When I unticked the layout scaling I got the dimension presentation I expected. When I unticked the model scaling nothing changed though.
I need to read the help file, draw and dimension a simple cube and see how all the settings and styles interact. Another Covid-19 lockdown project!
Thanks for you help