Having said that, the Gmail server might be worth a look, I just wasn't sure exactly where that gets changed.
Do I just need to change the From Address (even though the issue is with the To Address), ie is it just a spoof type thing for the To Address, by way if using the GMail SMTP info?
So it can be any GMail account / email address used here as a means to an end, even though no emails are actually going to the GMail address?
Or have I completely misread that?
I think I may have, as the From Address is currently set to the email address field filled in in the form:
<?php echo ((isset($_POST["Email_Address"]))?$_POST["Email_Address"]:""); ?>
And looking in the server settings, they are just currently:
Email object: mail
Character set: default
All the rest is blank / empty - I assume that's fine, or else it wouldn't be working at all.