I Have had a slow puncture for a couple of weeks now and every couple of days was putting a bit of air in it. Took it to a tyre place today and they say the hole in it is too big to repair. Surely if the hole was that big it would go flat overnight? are they lying to me?

Not if there was something in the hole (That which made the hole?) stoping the air leaking out quicky.

Repairs are a bit of a dark art and a lot of places are very strict on the guidelines adhered too when performing a plug

depends where the hole is, it might be near the tyre wall. Why not put the spare on and take the 'punctured' tyre to other tyre dealers for a second opinion. Could be they just wanted to sell you a new tyre.