Hiya, mines from the GP too, it's cheaper because pharmacys & chemists do it on a "for profit basis" where as the doctors surgeries don't (providing your registered there).
Also if your doctors a bit of an old hack, ask for an appointment with the nurse or person that specialises in Diets at your surgery. (it's actually a nurse at mine rather than a doctor). They'll then check your blood pressure and do your weigh ins etc instead for you each week.
Just out of interest Pizzle which surgery do you go to coz you just exactly described a nurse that sees me too! lol
Hope this has helped.
Cath.