OK, thanks for looking, I'll try and think of a workaround.
This prompts my second question.
Before I added the server validation, I checked for the add to cart 'submit' being pressesed, and this allowed me to display the results.
After adding the server validation, if the validation fails, and the page is reloaded, however, in this instance, the 'submit' has not just been posted, so the results are not displayed.