Solved! Go to Solution.
If your mail app is configured as POP3, you won't be able to. You need to configure it as IMAP and then include the spam folder to be synced. It is not possible to change from POP3 to IMAP on the fly, you would need to delete the account and then re-create it.
Thank you for info / advice
Technology not as seamless as we assume
Belt and braces it is then