Sugar I've taken norethisterone the times I've needed a bleed inducing, there is also a similar medication called provera. Norethisterone is a contraceptive that is more commonly used to delay a period, ie if you are getting married and are due af you can start taking it just before you are due on and it will push you back. For those of us who need bleeds inducing it basically kicks your hormones into thinking you are at a certain point in your cycle, then when you stop taking it you bleed.
You can get it from Oxford online (and they are a proper pharmacy with real doctors) or I would go back to your gp and tell them they are an idiot! Or ask to see another GP who might actually know what they are doing. If you get it from your GP it is free (as it comes under contraceptives, which are free).