Require a solution, list of sequential number but ignore a number if it is out of sequence

Hi, I am working with a sheet of invoice numbers which generally are paid when the job is done, occasionally one does not pay at the time.
I use a formula to look for the last number in a column on sheet 1 and pass it to sheet 2 where another formula adds 1 to it for the next invoice number.
The new numbers are used if it is paid or not.
When an old invoice is paid the forula will pass that old number to sheet 2 and mess everything up if I don't remember to amend it.
Can the formula I use be amended to not count the old number if it is less than the entry above it?, or any suggestions for something else please.

I have attached a sample workbook which contains the formula etc.
Many thanks in advance.