Bosch Greenstar Highflow440/550CDi boiler heating wont come on t error code?

The timer on my new Bosch Worcester Greenstar Highflow 440/550CDi won't seem to work in that my heating won't come on. I have no mobile thermostat - could this be the reason? The display reads "t" & sometimes a t & a backwards t but there is no reference to this in the user guide. Only some of my radiators are getting warmed too. Any suggestions gratefully received!