Good heavens, someone out there who actually knows all about this issue! As it happens my 'How To' guide is somewhat different to the Microsoft script being more related to my hardware and operating system and was found at http://www.draytek.com/index.php?option=com_k2&view=...
By slavishly following this route it was found that the missing registry key mystically appeared in regedit so it is no longer an issue.
I have successfully made the desired VPN connection but it was a hard slog over a 14 day period to get a result with much misinformation from various sources and incomprehensive technical information supplied by Virgin Media Business which varied according to the representative spoken to.
Fared no better on the Virgin Forum where I was told the facility was for residential users only and that I should ring Virgin Media Business help desk who promptly told me that they couldn't advise at all on any 3rd party hardware.
What was my task? No more than downgrading the Virgin Router to Modem Mode and daisy chaining to my own router which is configurable for incoming VPN connections.
Unlike an FTTC setup with a Openreach modem daisy chaining into your own router where all the parameters are configured in the router. the VMB modem requires the PPP username and password to be present together with the Gateway IP address and Subnet Mask. So the physical interface between the two units is the same but configuration within the router is very different. Piecing all this information together was a long haul.
Remote users now come through with an encrypted connection of type L2TP AES-SHA1 Auth whatever that means but it all looks pretty secure to me. I guess it was unrealistic to expect too much help with this project but I though there would have been many other VM users who would have wanted remote access into their devices.
With grateful thanks to camieabz for responding. Case closed and I can now move on with other work.