Potterton Suprima 30?

I have a Potterton Suprima 30 boiler fitted with the latest PCB board. British Gas have just replaced my pump, 3 port valve, tank thermostat, boiler sensor and PCB but the fault continues. When the Hot Water is calling for heat the boiler will stay on constantly for about 30 - 40 minutes (From Cold). Then it begins to cycle..switching itself off and on. The tank thermostat is still calling for heat so the problem would appear to be temperature related and something within the boiler which is over riding the tank thermostat and shutting the system down. This always seems to happen when the tank heat is at approx 50 degrees and the thermostat is set to 60 and boiler set to just under maximum. Does anyone have any ideas at all please ?? British Gas coming out again on Monday afternoon ...thanks