llihp "I'm trying to work out if this is an issue with Virgin"
It would seem that VM's spam filters have found something suspicious with these forwarded emails and so are flagging them as spam. Those filters are updated all the time so it's probably a recent change that has caused them to now be flagged as spam.
The first thing is to make sure you have followed Fasthost's advice regarding setting up DMARC, DKIM and SPF records for your domain. This might be useful: https://www.fasthosts.co.uk/guides#/articles/what-is-dkim----7158dd5d-175c-4d00-a9c3-6bff0cce6c92
However, the real issue is that although forwarding your own domain emails to your Ntlworld account is a cheap and cheerful way to manage your emails it is always likely to be prone to problems - as you have now discovered. In the past, some people using the forwarding option have posted here that VM actually blocks their private domain forwarded emails.
You would be much better off making use of Fasthost's webmail or email hosting options to manage your private domain emails.
Finally, it's worth noting that VM stopped issuing new email accounts almost 4 years ago so the future of the VM email service is unclear. Best not to rely on it to manage your private domain emails especially if you use the domain for business purposes 😉
Coenoby