A P45 only coves the year that you are working, otherwise you need to apply to Revenue for a tax cert. I'm not sure of an answer here and I can understand why you would not get one. You should contact the relevant section in Revenue and see what response you get. I would expect a P60 and a P45 to be the same based on the dates you have provided.
Separately, I used a final payslip last year with my med 1 claim and they accepted that as it captured the full years earnings.