I have just contacted JG regarding this problem and apparently this is a default setting on the thermostats to stop the valves/pump seizing if the system wasn't used for a while. This can be overridden by changing D07 1 to D07 0 on the thermostats. I've now done this on all thermostats so will see what happens next Saturday.
You need to go in to the settings menu, I'll list the stages
Press and hold the left, right and tick evenly with 3 fingers until 00 PSU is displayed.
Use the up arrow to 49 then press tick, 0 S01 will be displayed, press tick twice until 0 d01 is displayed.
Use the right arrow to D07. It will probably say 1 D07. Then use the down arrow to change 1 D07 to 0 D07
Then press and hold the tick. It should now be back I the main display.
You need to do this on all thermostats.