Your employer gave a supplementary Income Levy Cert for the extra income, but they should also have given you a supplementary P45 too. You should contact the payroll and request it. Or you could ask Revenue if they have received supplementary P45 details, if you prefer not to contact your former employer.
You should add the supplementary P45 figures and the original P45 figures and enter that as 'Gross Amount of Taxable Income' at question 3. You should also add the supplementary Income Levy Cert figures to the original Income Levy cert figures and enter that as 'Gross Income for Income Levy'.
As the income is Paye Income it shouldn't be entered at Question 18.