Given the week we are having now i wouldn`t be suprised to see it drop.
Don`t forget we have only had about 3 0r 4 days of hot weather this month,most of this month as been pretty average with one or two days being very cool indeed. A very average June for the uk this year.
Might only have been 3/4 days of 'hot' weather, but most of the rest of the tme it's been warm, and nights not exactly cool. Even a max of 21c and min of 12c should be nudging the CET up from high-14's. Tne hot days (27/14-ish) should be driving it quicker. But in the last 2 weeks, it's only risen from 14c to 14.9c (Metcheck's 'CET' has gone from 13.75c to 15.18c, still below what I'd expect, but 1.4c up in that time)