Alan White wrote:
What did happen to it?

From Roehampton University website
(http://www.roehampton.ac.uk/weather/...p#frosthollow: )

"1. The Chiltern Hills and North Downs

Since London lies in a basin, valleys tend to run towards the capital -
hence the potential interaction between the frost hollow and the
heat-island. The 'donor' areas for the cold air are the hills around
London - notably the Chiltern Hills to the north-west and the North
Downs to the south. The dip slope of both Chalk outcrops runs towards
London. Dry valleys in Chalk are most appropriate 'reception' areas for
frost hollows. On a calm, clear evening, cold air will run down these
shallow valleys towards the edge of London. The most famous frost hollow
probably is that at Rickmansworth in Hertfordshire, where a dry valley
from the Chiltern Hills meets the larger valley of the river Colne.

Confluences of valleys are effective frost hollows because two streams
of cold air meet and therefore become trapped in that location. This
effect is accentuated at Rickmansworth by a railway embankment across
the end of the dry valley. Air frost has been recorded in every month of
the year here, comparable to the Scottish Highlands. On 29th August 1936
the temperature climbed from 1.1 C at dawn to 29.4 C within 9 hours, the
largest daily range in temperature recorded in Britain.

To the south of London, dry valleys running northwards towards Croydon
have the additional favouring factor of a northward aspect. The
Chipstead valley has similarities to Rickmansworth, even including a
railway embankment which acts as a dam, above which cold air can 'pond-up'."

--
Joe
Wolverhampton