A note of encouragement: first 10 days of June 1975, average CET: 13.67
First ten days of 1995, 12.40 (and as far as 15th only 12.12!)
First 10 days of June 1990 12.34 (and only 12.21 to 15th).
Best of all, first 10 days of 1989 only 10.35.
Now, those of you who know these Junes and their following summers will understand all too well how not only summer can turn around, but June itself too. June 1975 is the odd one out here, as it had already switched paradigm by 10th. But 1989 and 1990 in particular are worthy of note- they kept getting colder to mid month.
Apart from 1990 these Junes ended comfortably above 14. Of greater interest were the summers generally, with August starting hot.