"What would you expect a decent broadband speed to be when it's tested using speedtest.net?"
Impossible to answer as it will vary hugely depending on how far you are away from your local exchange.. but practically speaking anything over about 2Mb/s is plenty enough for day to day browsing and anything over about 4Mb/s will be enough for most online streaming services (such as Iplayer)
If you put your details in this site
http://www.broadband-.../broadband-line-test/ it will tell you what sort of speed to expect on your line, as long as your speed is close to that then not much you can do.
(ann... assuming you are paying for an upto 8Mb service then nearly 7 is pretty good)