My alu Rush had a similar problem – – just a loose rivnut as others have said.
If it's an alloy frame it's an easy fix if you can get any bolt that is in the boss out (I managed to force an old hacksaw blade between the lip of the boss and the downtube which held it enough to unscrew the bottle cage bolt). A decent shop should have the proper tool, otherwise you can do a DIY bodge if it's just a bit loose – instructions here (2nd one down).
http://www.m-gineering.nl/tipsoldg.htm
If it's an alloy frame I would be tempted to sort it myself or get LBS to do it particularly if Evans is a long way away – a carbon frame would be a different matter though.
Hope this helps – doubt it is a massive problem in an alloy frame at least and Cannondale warranty is very good in my experience.
(EDIT – Carbon frames are different as the bottle bosses tend to be bonded into the frame rather than rivnutted – potentially more serious)