I received a reply from VM quite quickly (just after the Bank Holiday). Here's the email, and it seems helpful and to the point. I'll be calling customer services and requesting my modem ready for the upgrade. I suppose if they install it FOC no point in trying to do a DIY job, although I am quite handy with computers etc. Hope this helps.
"Thanks for getting in touch with the Virgin Media Support team.
Thanks for your patience waiting for us to reply to your email and please accept our sincere apologies for the delay. Rest assured that we are working very hard to answer every customer enquiry, and to ensure that our reply times reduce back to normal as soon as possible.
Thankyou for your enquiry about upgrading from 2 to10 meg. The best option would be to get a stand alone modem installed if you wish to upgrade to 10 meg. This can be arranged free of charge by calling Customer Services dept. Once this is done, you can register for the upgrade by going to
http://www.virginmedia.com/gofaster/ the upgrade will be taking place between May and September, depending on which area you live in."