I just got off the phone to a virgin tech, a real nice guy with zero messing around, he confirmed that the sb4100 was not up to the job.
The new one will be the EPC2100, and will be sent out today.
We have been on the xl package for just over a year and I put the slow speeds down to my area and thought nothing else of it.. (I was first told by virgin when I upgraded that my modem was fine for the 20Mbps service..)
That was until I found the info on here...
great site
I will be also getting the money for the last years so called upgrade refunded. It's about £20 but I could do with it more than Mr Branson
(I got my girlfriend to phone, a nice phone manner helps

)
I will post back when it gets here and let you know if my speeds are any better.. I'm sure they will be.
Thanks all, for the tips and advice, I hope you other guys with this same problem get sorted out.