If I remember correctly then the digital thermostat / programmer I've
got just acts like a switch. there is no + or - and there is not even
the conection for the 3rd wire that you get with the old mechanical
thermostats.

If it's like mine I dont see how you could wire it incorrectly in the
first place. Even if it needs an AC supply then this has no 'polarity '
does it !

Simon