The celebrations of fertility the Father Sun mating with Mother Earth, don't take place on the Summer Solstice, they take place at Beltane, 1st of May.
Toward the end, the car used to race to catch the killer changes when DCI Tom Barnaby and Sergeant Dan Scott reach their destination. This is the same dark green 5-door Rover 45 first seen in "The Green Man" (#S7:E1).
(At about 1:21:45) The "drowning victim" blinks his eyes slightly in the sun.
Unfortunately the 'Barrow' looks all too much like what it is; a heap of stuff with not very freshly applied turf on it, in the middle of a very flat field. Real barrows don't look like that.