Bolier does not heat up hot water?

I visited my parents over the weekend and noticed that there was a loud grinding type noise coming from their heating system. It was coming the pump. Then all of a sudden the hot water stopped working closely followed by the central heating system. So I drained down the system and replaced the pump (old: SMC commodore 130-45 Speed Position Minimum with new: WILO Gold 130mm / 6M Speed Position 2) and the isolation valves on either side because they were weeping. Then I reconnected, filled up, vented pump and radiaitors. The central heating worked fine but no hot water. The bolier fires up when hot water is switched on but then after a short while (30seconds) it switches off. I have two off 2-port motorised valves, one for the hot water leg and one for the central heating leg. Both are working ok. The boiler is a glowworm space saver 75. I have also replaced the thermostat on the hot water tank (old: Drayton CS1 with new: Drayton HTS3)
Does anybody know what I should do as my parents have no hot water.
BJ