Hi Mike,
Welcome to our forums and thank you for posting. I'm sorry that you've had difficulty using your mobile with a new SIM, we'll do what we can to advise accordingly.
If our support team has advised that this handset is not locked to our network, I'm confident they've provided the correct information. If the phone was locked, I would expect you to see an error message indicating the need to unlock the handset when you start it up. As you're not provided with the opportunity to enter your unlock code, I'm wondering whether the issue is due to SIM compatibility.
As this is an older handset, it may not be able to recognise newer standards - e.g. if it's a 2G phone but you're trying to use a 4G SIM. Have you checked what type of SIM you're trying to use and what this particular device is able to recognise?
Regarding the Virgin logo, this doesn't necessarily mean that the phone is still locked to us. I do believe you can remove this by typing #67705646# from the dialling screen of your phone (this may just be for certain models).
Thanks,
Rachael