Can't work the heater timer?

[Zizzy]

The parking heater worked brilliantly on our first trip this weekend but i couldn't get the timer working. I set it for Saturday at 7.00; duration: 2 hours and a little clock symbol appeared but the heating didn't come on the next morning. My partner tried for Sunday morning with the same result - nada! I think we must be missing some simple but critical step. I have read the manual and could see where I went wrong. Any ideas?