I got Spark to connect to my btinternet account OK by resetting the server ports to 993 and 465. I cannot send any messages at all from Spark on the iMac. It would seem that my being able to send a message to my iCloud account from the iMac is an Apple peculiarity. Spark works a treat on the MacBookPro - no problems with sending messages at all. I wonder if its an OS problem (11.5 works; 10.15 doesn't)?
Oops (again)! Spark made all the right noises when I sent the message but later it declared there was a problem and I needed to edit the message. I was, however, able to send the same message in Apple Mail without any issues. The message from Spark never appeared on webmail; the message from Apple Mail did.
Thanks, liquorice, for your advice. The problem has mysteriously disappeared . It was there this morning at 0800 but it was gone by 1330. I seem to recall someone suggesting that it might and so it did. Hurrah.