The reality of our government is that PMs have to deal with 'Events dear boy events' and do so mainly with the guidance of senior civil servants who provide the information on which they base their decisions
Hello Caroline 
I think that was probably the case in years gone by - as epitomised, of course, in that great series 'Yes Minister'. Ministers were in charge of departments but the civil servants guided. However, there seems to have been an increased trend in recent years for 'special advisers' to be appointed who appear, quite often, to have been the power behind any utterances from No 10 - one of the most prominent/notorious depending on your point of view being Alistair Campbell. Is this trend continuing and is it a good thing or undemocratic? - after all, these people seem to wield enormous power but are unelected.