The answer is '39 Inches in a Metre'.
39 Steps (movie by Alfred Hitchcock)
3 Barleycorns In An Inch
3 Barleycorns In An Inch
Nine threes are 27
I Saw 3 Ships
16 Feathers in a Shuttlecock (Standard Badminton)
This is a Ditloid. The answer is 26 Miles in a Marathon. (approximately)
6 = Henry (the) Eighth's Wives
13 M of N = 13 moons of neptune.
568 M in a P = 568 millilitres in a pint.
4 musicians in a quartet
Millimetres in a Yard