this may sound stupid but how can you tell if fondant is the right consistency for piping, i keep setting up the piping bag only to find it wontstay on the cake, any advice on piping would be grateful

Fondant really isn't considered a piping medium. Mostly it is used by rolling; either by hand or rolling pin; cut w/cutters into shapes etc.

Do you mean you are piping royal icing or buttercream onto a fondant covered cake?

it is royal icing onto a fondant cake for borders and writing, sorry i didnt make myself clear

You need to make sure its nice and thick Thick enough so it holds its shape really well, but not so thick it gives you carpel tunnel while trying to pipe