Problem with Ferolli Modena 102 Combi Boiler?

Hi,

I've got a Ferolli Modena 102 which has got water constantly coming from the overflow pipe. The pressure is sitting at around 3 and the filling loop is disconnected so it seems to be drawing water in from the mains. Could this just be the expansion vessel that needs replacing/recharging or is it more likely the exchanger is perforated? What pressure should the expansion vessel be recharged to?