According to the RFC:

https://datatracker.ietf.org/doc/html/rfc7208#section-5.3

"a:savella.carfax.org.uk" is going to look up the addresses of
"savella.carfax.org.uk", not do a TXT query for an SPF record at
"savella.carfax.org.uk", so why do you believe this is needed?

I was just going by Gmail’s response, indicating that it had attempted to check ’savella.carfax.org.uk’ rather than just ‘carfax.org.uk’. That suggests that the envelope-from is actually ’savella.carfax.org.uk’, although I don’t know whether this is intentional on Hugo’s part.

Cheers,

Mike