Kingfisher Potterton boiler?

Hi. We have a Kingfisher Potterton boiler which works efficiently and is regularly serviced with the last one mid February. Soon after the service by British Gas we went away but left the gas heating on low. Soon after we returned we started to get a very musty smell from the boiler and I say it smells like fertiliser! British Gas have checked it twice and no gas leaks or carbon monoxide problems as we have two detectors in the kitchen where the boiler is. The smell occurs when the boiler is on and with or without the water on. The burner seal was changed but we still have the problem. Has anyone got any ideas please as to what has caused this smell?

Angela
April 2015