Worcester 28cdi - DHW demand indicator light flashing

Help please, the DHW demand indicator keeps flashing fast. Only way to re-set is by turning on/off at operating switch. When fault occurs, there is no hot water obtainable and the central heating also does not work.
Fault occurs in various modes - central heating on only, hot water demand only, both central heating and hot water on. Tried altering the temperature on the dials - this does not stop the problem. Have checked the pressure gauge and made sure the flow is at 1.5 normal. Have replaced a thermostat, replaced the fan, checked flue for blockage. Numerous central heating contractors been called out but none have rectified the fault.
Indicator light is now going on at least 6 times a day and it is now beyond a joke. Boiler is approx 7 years old since being installed.

If you look at the manual you'll see it say that it's either
temp sensor
wiring harness/connections
gas valve
printed circuit board.

INtermittent faults are often difficult to find.

Call wb, for fixed fee repair, bet they have it working in half an hour. Good boiler, my first guess would be pcb not seeing the flame sensor. Shame you are so far away