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.

Copy and paste the "non-working" cell elsewhere and it continues to be wrong
- even if overwriting a "working" cell.

Copy and paste the "working" cell elsewhere and it continues to be ok - even
if overwriting a "non-working cell".

Any thoughts very much appreciated.