March 1957 stands out as being the mildest in the CET series which started in 1659, and as mild as this current March is, the mean anomaly of which stands at +2.48°C on the 12th, it will have to do a lot more to surpass the mean temperature of 9.2°C of March 1957, which was +3.53°C above the 1960-1991 long-term average (fig 1). Here are the daily surface charts for the month courtesy of Wetterzentrale (fig 2). In fact even 60 years later, the month still holds claim to three of the warmest March days in the CET series, the 3rd, 11th and 12th. The MWR explains how a pulse of warm dry air from Spain on the 10th, produced a period of Summer like weather that lasted to the 13th. Temperatures at several places in England and Scotland reached 70°F on the 12th, at Haydon Bridge (Northumberland) they reached 74°F (23.3°C), and unusually 69°F was recorded as far north as Cape Wrath.
As you can probably see from the daily charts (fig 2) the country was flooded by mild south or southwesterly for much of the month, the mean pressure chart for the month shows a negative pressure anomaly of -17 hPa at around 30° west and 50° north (fig 3 & fig 4), with higher than average pressure over Greenland (+11 hPa) and Scandinavia (+6 hPa).
This of course resulted in unseasonably warm temperatures, not just affecting the British Isles, but also across much of western Europe as well, with a positive warm anomaly (+4°C) over eastern France and Switzerland (fig 5).
So March 1957 ended up with very few stations reporting an air frost, and ended up warmer than the following April, quite an exceptional month.