I was dropping ValueType elements, so I have changed and dropped a Property on - all works now.
If anyone else comes across this in the future, it is necessary to change the Type by opening the Property element and in the dialog under Properties - Property selecting a Value Type. Confusingly, the Type in the Properties window (not the dialog that is visible when opened from the element in the Project Browser) is not editable. That's too many uses of the word Type in windows/dialogs labelled Property but doing different things for my mind to cope with!