Government is big, with around 100 ministers in place at any one time, and it’s not always obvious who is responsible for what . I’m sure you already knew that the Minister for Plant and Tree Health is the Minister of State for Minister for Rural Affairs and
Biosecurity, but are Green Jobs DEFRA or BEIS? Who is responsible for the Fire service?
The List of Ministerial Responsibilities explains exactly who is responsible for what; in general, ministers will only answer your correspondence if it relates to something listed here as being under their control; it’s fine to write to the Secretary of State but they will generally forward your letter to the relevant junior minister.
The list also gives phone numbers for the private offices and correspondence sections within departments.
It is available online at: https://www.gov.uk/government/publications/government-ministers-and-responsibilities.
Be aware, however, that the list is not updated with every reshuffle and so you should check here to make sure that the person named is still the Minister: https://www.gov.uk/government/ministers