louisxiv writes
Hi
Probably a silly question but how do you guys decide when it's safe to
assume there'll be no more frost?

Experience.

I make a note each year of the early and late frosts, and over the years
this has shown me that I cannot be sure of being frost free until the
second week of June, and the first frost in autumn will probably be in
October.

There's no real substitute. Temperatures vary not just with whereabouts
in the country you are (in summer the south is warmer than the north; in
winter the west is warmer than the east), but also altitude and the
layout of the surrounding land - cold air sinks, so you may have frost
in the valley but none if you are part way up the hillside.
--
Kay