This information is not stored anywhere within the system or database. This RTI ID is generated each time you generate an FPS, and stored in temporary memory and attached to a XML file that is sent to HMRC.
The XML file contains details of the workers pertaining to the FPS you are sending.
This XML file also contains the RTI ID of the workers in the FPS, and these IDs are unique to the worker.
Furthermore, after the XML file has been sent it gets archived in a folder, in the PAYE directory. If you have access to the below folder, you will be able to see each of the workers RTI ID:
Network Drive:\Pay&Bill directory name\Pay and Bill\ReleaseXXXX\PAYE Internet\Archive
There will be .xml files for each and every FPS you have sent. So if you can remember, or identify which FPS has the workers you are after. You will be able to go into the relevant .xml and identify the ID for the worker. The employer ID will be in a similar format to below:
<PayId>EmployerID WorkerID SequenceID</PayId>
Eg: <PayId>EMP01 W12345PA 0000000001</PayId>
This ID represents the worker. This should be above details of payment made to that worker.