martin j gurgled happily, sounding much like
they were saying:

Got to get to Harrow on the Hill, Tuesday evening and Journey Planner
says 12 mins for the Met Line from Finchley Road to Harrow on the Hill.
How much should I add to get a realistic figure?

Most of the time, that's perfectly realistic. Fast (typically Amersham) &
semi-fast (typically Watford) trains will, of course, be considerably
quicker on that stretch than all stations (typically Uxbridge), since
they don't stop at the intermediate stations except for some at Wembley
Park.