Apologies, I have just solved the issue. For reference here is what I think the problem was:
I had an existing dynamic <option> in the select. In the past (and before using the Webassist function) I have been able to apply a repeat region to the <select> and then do the dynamic "if selected" function. However using the Webassist function to perform the same approach seemed to cause the issue.
By removing out the entire repeat region around the <option> and then using the Webassist function I was able to get this working. It seems that the Webassist function gets confused if something is there already ;-)