Radiant RS24E. Error light 14, lack of water in system?

Radiant RS24E. Boiler has been in use for 8 years with no problems (result!!). I use the boiler for heating only, so it is switched off over the summer. On turning it on this year I have error light 14 flashing, which indicates a lack of water in the system. The pressure gauge says there is 2.5bar of water pressure in the system. I have drained and refilled with no effect. Any ideas?