Does anybody know if the Vigor2800G will maintain line speeds of greater than 8Mb? Im thinking of going to Sky where the line speed is between 10 - 20Mb and was wondering if anybody had any experience?

I would say to expect problems! at most exchanges Sky run ADSL2, and the 2800 doesn't handle this well. You can set the 2800 to stick to G.DMT for maximum stability, but then you'd loose the higher speed.

As has been said, the 2800 routers do not play nicely with ADSL2+ which is what the majority of connections now are.
Also Sky will not allow you to use another router - so you do this at your own risk. To that end I believe they will not tell you the logon name / password needed in the router, but send one pre-configured, so if you go down that route you have to figure out how to get those details.