When I make a big cake I cover it in  buttercream, I have tried loads recipes which never turn out right. Does anyone have a foolproof recipe for a large amount of buttercream, thanks

define 'never turn out right' and are you making american type buttercream that is powdered sugar, fat, liquid and flavoring? or what other kind do you mean? do you have a good recipe for a small batch of buttercream? you should be able to multiply it out for good results --

No just making uk version with just butter and icing sugar, it either turns out too runny or to thick or the last one I did look liked it had curdled

Well, that sounds like American butter cream. Try and add some liquid like milk to it, little amounts at a time when it's too thick. It might have been so thick that it looked curdled. Or maybe the butter wasn't soft enough? If it's too funny simply add more sift icing sugar until it has to right consistency.