The highest UK-wide monthly temperature range for May
appears to be 38.4 degC (actually 69 degF), from minus
5.6°C to 32.8°C in 1944.

This month, assuming the extremes are confirmed as
minus 6.3°C at Tulloch Bridge on the 18th and 31.4°C at
Herne Bay on the 27th, the range will be 37.7 degC.
With lots of stations not reporting till well after the end
of the month (and then being generally unavailable to the
likes of us), that record could yet go.

May's record is one of the smaller ones. March is
probably the biggest: 46.7 degC(*) in 1965 and 39
degC as recently as 2001. December 1995 managed
42.7 degC, and November 1985 38.6 degC. August
2003 easily set a new record for that month with a range
of 37.9 degC (based on limited data).

(*This uses the published value of 25.0°C at Wakefield
on the 29th which was so far out of alignment with adjacent
stations that it is very probably wrong. Even without the
Wakefield max, though, the range was still almost 46 degC.)

Philip Eden