Wonder also whether then structure of modern episodes means the Doctor is on screen for more time (hence more recording time for the relevant actor) than in years gone by?
Right up until the end of the Original Series the bulk of each episode (excluding location scenes obviously) was shot in one evening with the plugs literally being pulled in the Studio if everything wasn't done by 10pm. The rest of the time was spent in a Rehearsal Room basically learning each story as if it was a play so as much of it could be recorded in one take as possible. So most of the time it was a 9-5 (or less) job.
Now beyond an initial read-through the rest of the time is spent actually shooting the episodes, mostly on location, for long hours.