I have 24 rows of 12 cells, starting at row 5 and then every 20th row after
that (i.e. D5:O5, then D25:O25, then D45:O45....up to D465:O465).

I need to find the last entry (a cell that does not display a zero) in each
of the 24 rows (which may differ from one row to the next) and then do a sum
to add each value together.

For example, if the last entry in D5:O5 was £10 in F5, and the last entry in
D25:O25 was £12 in G25, then the result would be £22.

What would be the best way to go about this?

Many thanks in advance