Many thanks for the link. Reading through it seemed that a proper Factory Reset was needed. VM had rolled back the firmware from that which had the alien SSID but a factory reset was needed to re-install. Mind you it took me ages to get factory reset to work. Holding in the paper clip for minutes, then turning power off then repowering with clip, didn't do it.
In the end a 'soft' factory reset from the Webpage GUI got yhr defaults back and hey presto no Virgin Media SSID
Strange their call centre bods know nothing about this
Thank for your help, I'll keep my eye on it and hope it doesn't re-appear.