Gmail and SPF

I use Forte Agent to send email, via Virgin's mail servers, with
replies forwarded via my own domain email address.

I have set up the Gmail app password, which has been working
fine, but am now getting bounce messages like this:

Action: failed
Final-Recipient: xxxxxxxxxxxxxxxx
Status: 5.0.0
Remote-MTA: dns; gmail-smtp-in.l.google.com
Diagnostic-Code: smtp; 550-5.7.26 Your email has been blocked because the sender is unauthenticated.
550-5.7.26 Gmail requires all senders to authenticate with either SPF or DKIM.
550-5.7.26
550-5.7.26  Authentication results:
550-5.7.26  DKIM = did not pass
550-5.7.26  SPF [cdixon.me.uk] with ip: [84.116.50.34] = did not pass
550-5.7.26
550-5.7.26  For instructions on setting up authentication, go to
550 5.7.26  https://support.google.com/mail/answer/81126#authentication ffacd0b85a97d-37d4b989e98si2592841f8f.501 - gsmtp

The IP address varies in different messages, all allocated by
Virgin's mail server, and clearly not unique to me.

I have read the various pages of instructions, including one on
my host:
<https://www.heartinternet.uk/support/article/how-do-i-add-spf-records-to-my-site.html>

but find myself totally unable to understand exactly what to do.
It also seems like trial and error is not a good way to go, if I
correctly understand that updated entries can take up to 48 hours
to propagate.

If I send directly from Virgin's online mail page, there are no
issues.

Chris
--
Chris J Dixon  Nottingham UK
chris@cdixon.me.uk  @ChrisJDixon1

Plant amazing Acers.