How to add the next number in a series when another cell changes

I have a column of sequential numbers (In random order) and I need to be able to add the next number (+1) into that column next to the cell in the next column to the right when the value in that cell changes from empty to a value selected from a drop down.

See attached the values are not actually numbers as you can see (UK328 etc) but I can make a new column with just the numerical value if required.