sorry, yes frost stat will activate pump on demand. as long as there is a pipe stat, this will cut the circuit and switch boiler off, if no pipe stat is fitted, the boiler will not shut down till limit stat is reached, if like you say the boiler is 7 year old,you can`t start questioning the way its fitted if its worked all these years, unless someone has tinkered with it. Outside worcester boilers have the frost stat fitted internally as standard, and require the switched and common live to work them. I have never had an issue with a frost stat packing up,so im not sure what the consequences would be, as frost stat is designed to overide all other controls