The web form is now working again after failing silently for an unknown period (could have been days/weeks/months!). I am guessing the hosting provider made a change (possibly when forced to PHP 8.x) that stopped the mailer working using the php mail() function.
Apologies to those who have tried to contact me – I wasn’t ignoring you!
I only became aware of this today when someone mentioned via social network while letting me know my weather station was having issues.