If the pressure is remaining OK, the chances are it's the boiler's flow switch not picking up hot water flow, a blocked domestic hot water heat exhanger or jammed/blocked diverter valve. Easy to check these - first run a hot tap and see if the boiler lights up.
If it does, check the flow going to the radiators (far left pipe out of the bottom) to see if it's getting hot.
If it does, the fault is either the diverter valve or diverter motor. If it doesn't, but the boiler is on, then it's likely to be a blocked heat exchanger.
If the boiler doesn't fire at all, then the flow switch is the most likely culprit!
Lots of people have this problem and a lot of the time the Flow Sensor has been found to be faulty and has to be replaced.