"chriscl" wrote:
Occasionally the = sign is not recognised as indicating
a formula, rather it appears as part of a text constant
i.e. typing =1+2 into a cell results in "=1+2" (without
quotes) appearing in the cell. In other cells it results
in "3" appearing as expected.

This (the text result) will happen if you put a space
before "=". Is that perhaps what you are doing?