Saunier Duval Thelia 623 has hot water but no heating help?

For the last 4 weeks i have had the boiler on Summer mode ie just used fof hot water which is ok , but when i recentley switched it to winter mode ie slid the slider to hot water and central heating. I don't get any heating, the pressure on the boiler is at 2 bar , the boiler fires up ok when i turn on the hot water taps but when i turn it on for heating nothing happens , i am doing this manually and made sure it is not on the timer etc , has anyone any ideas how to get the central heating working cheers

Jamie
August 2008