Thorn apollo 15/30B boiler?

Re last question:
I have checked and cleaned out the heat exchanger and flue and all seems to be clear. Now I find that with the bioler internal cover off, the gas valve turns off after a short time. I have changed the thermocouple, and metered out for continunity the over temperature stat. and all seems ok. I have tried the old thermocouple with the interruptor linked out and the boiler seems to run ok for a while before I turned it off incase there is some other problen that the system safety is cutting out on. Please can someone give me some suggestions. This is a balanced flue boiler with out any fan assist.
Thank you in advanced