Finally decided today I wouldn't mind 50meg...gave VM a call about upgrading. Thought it would be nice any easy, they add the extra £15/month or whatever the difference is to our bill and send me a self install modem kit and that's it...but no.
First of all I have to pay £30 for an engineer to come to my house and make some "modifications" to the box on the outside of the house. Then when I put the new modem in, I have to pay £20 to get it registered. Although I was told the extra monthly payment is only £8 rather than £15, which is pretty good.
So £50 to upgrade my service, which is going to cost more per month anyway, doesn't see very reasonable to me. So I'll be staying firmly with 20Mb for the mean time