Ok, I've had a chance to look into this some more.

If I put Biff's formula into a column next to a column of made up numbers
and spaces, it works.

When I put it into my sheet it returns 0. Presumably, this is because the
(MATCH)-1 part evaluates to zero.

The formula in the cell to the left (in column I) is:

=IF(A3="End of job",
"end",IF(ISERROR(IF(ISBLANK(C2)=FALSE,C2+H2,""))," ",IF(ISBLANK(C2)=FALSE,C2+H2,"")))

I tried inserting another column with =I1 etc in it - same result.

Hope this helps,

MM