Vaillant EcoTec Pro 28 - Hot water but no Heating?

Hi All

I have the error s31 coming up on my boiler. This seems to point towards the thermostat being faulty but i have replaced that to no avail.
The hotwater works fine & when you hold in the + & - together you can force the heating to come on for about 5 mins then it just turns off again.
Does anyone have any ideas on what else i could try?

Any help would be much appreciated,