The last scene of the series (not counting the Christmas Special) is a call back to the very first episode of the series: Doc Martin again takes up the "For Sale" sign posted in front of his surgery, and heaves it over the cliff.
Mrs Tischell's line "The heart wants what the heart wants" is a take on Woody Allen's infamous line "the heart wants what it wants" during his separation from Mia Farrow for having an affair with her daughter.