Potterton statesman won’t stay running

Hi,
Looking for some advice here on a Potterton statesman inside oil boiler that stops running after about 7/8 mins. Bit of background first. On the front panel there is a switch for HW only or HW/CH. My system is not plumbed to supply HW only, so switch is always set for HW/CH. My 2 year grandson decided to try the other setting only I didn’t spot it till later. Anyway the boiler now turns on as normal but thermostat trips out befoyyre the first rad gets hot. The inlet pipe to nearest rad gets hot but then the boiler turns off. I can turn up the thermostat but it goes again within a minute. I have replaced the thermostat and limit stat but no change. The pump which was replaced 3 years ago now runs with a unusual whirling sound. I can hear it running even after the stat trips out. Wondering has it been damaged by overheating and not able to move the hot water? Is there another valve upstream of the pump I can check?
Any advice welcome