CB28 overheat problem. The diverter valve diaphragm has been replaced recently, so has overheat thermostat. Boiler trips out with red(A) led flashing red at approx 1 sec interval, i.e. overheat stat fail. Thermistors in HW and Primary circuit check ok, 10k when luke warm, 3.8/4k hot. Pump seems ok, very hot water at pipe to CH 1m away from boiler within 2/3 seconds. Gas flame does not seem to reduce as CH warms up, Very high with hot tap full on, reduces to level of ignition sequence when tap off and CH on. DHW switch checks ok, turns on/off with hot tap, boiler turns on/off in sympathy with no CH on. Average DC voltage across modulating coil of gas valve starts at 5V for ignition, goes to 11V at full HW and drops from 6V to 2V as CH heats up, but flame level does not appear to drop from level at 5V. Modulator coil seems free and diaphragm looks fine. I do not have a gas pressure gauge. Is the gas valve faulty? PCB appears ok, no dry joints etc. I am retired electronics engineer.