I live in a large town, yet almost invariably record temperatures
several degrees below their predictions, on clear nights.

I know Cheltenham fairly well having lived nearby in the 1970s.

Cheltenham is bound to be in a cold pool on clear nights simply as a
result of draining of cold air from the adjacent hills. I have in
fact on several occasions experienced noticeable katabatic winds
descending off Cleeve Hill.

Although scarcely qualifying for the term "frost hollow" the principle
is the same.

Jack