On 13/08/2010 10:52, Andy wrote:
To test a bit more thoroughly I tried copying only your code to a
brand new workbook and it still seems to give the same problem
although the invalid cells are different.

I typed 00:00 into the first cell of another workbook, copying it down
to 23:00 then copied it over to the coded workbook. Same formats again.

Andy,

Try putting 0:00:59 in a cell.

Drag down. Select the resulting range.

Paste into C11.

You shouldn't get any errors.