Thanks for answering my query.

Your suggestion would definitely be a way to input public holiday data for many years ahead, and use that to modify the data in a timesheet worksheet. I might look into doing that.

As for a formula for generating the week number of the second week for any fortnight during the year, I've since discovered this seems to work:

=ISOWEEKNUM(A2 + (MOD(ISOWEEKNUM(A2)-1,2)=0)*7)

Simply having a =NOW() formula in cell A2 to refer to, another cell with the above formula seems to return the correct value.

Cheers.