XS    S   M   L XL
PTP
15" 16" 17" 18" 19"
Waist 12" 13" 14" 15" 16"
Hip 17" 18" 19" 20" 21"
Length 32" 32.5" 33" 33.5" 34"

*Model is 1.68m, UK 6 (PTP 15.5", Waist 12.5", Hip 18.5") and wears size S.

If you are in between sizes for PTP only, it is best recommended to size down. 
If you are in between sizes for waist/hip, it is best recommended to size up. 

*In between sizes refers to your measurements falling between 2 sizes, example size S & M.