The fuel burner in my S type 2.7 diesel is supposed to activate whenever outside temperatures fall below 5 deg C but fails to do so. I have had the fuel burner itself replaced but this hasn't resolved the problem. It seems that whatever process should activate the fuel burner isn't working and my authorised Jag dealership say they don't know why and can't do any more. Any advice will be much appreciated

I don't have a S type but on my X350 with a 2.7 D there are over 30 fault codes associated with the fuel heater. 

Find a garage that can at least read these and ask them to what codes they can find.  Then write to Jaguar cars Ltd and ask why their authorised dealer is unable to rectify the fault.