Quick question, how can I tell if the batteries are out in the hot water temperature module?

Odd thing happened yesterday, where I had no hot water. Manually opened the valves and all OK, so set them back to Auto.

Went on to try and do an RF Signal check and when I pressed and held the button for 5 seconds, it flashed between green and red, not as it described in the manual.

Turned the heating off and just ran the hot water and all it got up to temp and it shut the boiler off at the appropriate point.

Also doubled checked that the standard cylinder stat is tripping the valves and it is, so the over temperature protection is working.

Any thoughts? It's been working fine for a year!