The supporting document used with BS 8007 was CIRIA Report 91, Early-age thermal crack control in concrete, 1992
R91 used to state T2 = 20 deg, as being conservative for the UK.
But R91 also quotes this:
"Knowledge of the temperature variations at the site is needed for the calculation of the early age
thermal movement and the subsequent daily and seasonal movements. In the UK, the
relevant information can be obtained from the Climatology Service of the Meteorological
Office"
