Yep EH8 is right, and that should be the general rule. The reason being that for abscences of up to 7 calender days you are legally allowed to self certificate. Now your company may require you to see their nurse/doctor on return but they are not allowed to request a sick note. If your actions are suspicious i.e. lots of 7 days off sick then, as if by a miracle, better on day 8, they can ask for future abscences to be supported by a sick note, which most would pay for.
The reason GP's charge for sick notes for less than 7 days, is that basically nothing much is wrong you, you have a bad cold or flu, but nothing to see a doctor about. You (or your employer)are therefore wasting NHS money, and the doctors time, so you get to pay for it.
In response to parris50 most GP surgeries are partnerships anyway, so it doesn't make a lot of difference who you make the cheque out to. Mind you I bet a few cheques made out to Dr.soandso escape the taxmans clutches