Hello Graham,
That's good to hear that the HouseMenu works with 4.x - I thought it was the main menu you had problems with, to remove Solpart menu from the containers and replace it with a drop down list, follow this tutorial:
http://skins.dnncreative.com you will see the home page completely validates.
Let me have a link when you are allowed to release it, I will be interested to see what you have created in CSS!
Cheers,