I'm not surprised by these stats as the 1960s was generally a very cool decade (also in a non-meteorological sense ) . Note that no year in the 60s had a CET above 10C - it was almost as if the climate temporarily regressed back to the Little Ice Age. All seasons were distinctly cooler. However, apart from 62-63, no other winters were massively below normal. I forgot to add that 1960s summers were particularly cool. 1969 was the warmest. If we had that kind of summer this year, it would be our coolest since 1993.
It looks like the mild or warm spell you described at the end resulted in a cold but unspectacular CET of 7.2. In fact, April 2001 was nearly as cold. It really shows CET averages can really mask cold and warm spells.