The transmitter light if flashing constantly?

I have a Worcester Bosch Greenstar 24i junior/28i junior, and the heating stopped working, i realised that the pressure was down, so topped up the boiler so that the pressure when cold was 1/4. After pressing reset the heating eventually came on but the green transmitter LED is constantly flashing fast, and the pressure is on only at just under 1. Does it need more topping up and why is the green light flashing, pressing overide doesn't seem to work?
Thanks

Claire
March 2011