OK, I have solved this one myself while waiting for a response.
It turns out that there is code in AppConfig.php within 'components com.sppagebuilder' that uses list_product_page_id to test if EasyStore is installed. e.g.
isEasyStoreInstalled(), 'list_product_page_id' => ApplicationHelper::getStorePageId('storefront'), 'single_product_page_id' => ApplicationHelper::getStorePageId('single'), 'is_pro' => ApplicationHelper::isProVersion(),
My guess was that list_product_page_id wasn't given a value when the SP Page Builder upgrade was installed.
I simply opened Global Configuration in Joomla 4, located SP Page Builder on the list and clicked on 'Save and Close'. Problem solved. The error message no longer appears and SP Page Builder pages open normally.
There is no mention of EasyStore in Global Configuration. I guess this wasn't meant to happen.