If you have been out of the UK for more than 183 days, and you satisfy certain conditions you may be entitled to claim your tax back.
If you are already set up for Self-Assessment you will need to complete the Tax Return accordingly. If you are not set up for Self-Assessment then you will need to be set up after you have decided that you have a claim. To do so you will need to write to your Tax Office and ask to be set up for Self-Assessment.
You can identify which is your tax office from the first part of your PAYE reference, the 'district number' which is shown on your P45 or P60 (e.g. 992) and by tracing that number from the list of local offices.