popsycal, the pressure should be around 1.5bar when the system is on and not below 1 bar when it is off. If it has fallen below 1 bar when off and you haven't just bled a radiator, then you may have a leak in the system (any leaky rads/pipes?). If you don't have a leak in the system then you probably have something wrong with your boiler (faulty pressure chamber/valve). If the pressure is low that could result in your rads not getting very hot, and if the pressure gets too low, your boiler will shut itself off and not start up. You can add water with the filling loop to get the pressure up to get it started. When you add water with the filling loop, do it slowly (while the system is on if you can). If you send the pressure above 2 bar by accident, bleed a rad until it comes down to 1.5bar. The thermostatic valves are nice, but notorious for sticking and leaking and not working properly.