re Worcester Boiler 9.24 Electronic. RSF?

The boiler is running in CH and Hot water mode. The
boiler temp is set at just 2 or 3 on the main control.
After drawing say two or three bowls of hot water the
burner extinguishes and does not re-alight when system cools. The Central heating sensor reads
10 to 13K at 20 to 25 degrees C. Drawing hot water again re-alights the burner, it stays alight and system runs normally in central heating mode. Is the pcb the more likely suspect?