Even though Sage exports text in columns A-C, you can use numbers, which are easier to work with.  Decide which column will be the thing that defines one JE.  In your example, that looks like the Reference column.  Assuming that is column J, use a formula in Column B that says that if Column J has changed, increment by 1, otherwise use the number from the row above.  Then in Column C, enter a formula that says that if Column J has changed, start at 20 (or any number, frankly); otherwise increment by 20 (also works to increment by 1).  The formula will need to be slightly different in the first row of data, or have an "IF" clause, to account for the fact that there is no number to increment from the row above.

Also, in Column A, there must be an entry on each row.  Again, it can be a number - I usually use 1 or 0.  Sage will overwrite it at the time of import, using the Batch# you're importing into.

Thanks Mate, seems very easy to understand and execute! Will try this out