Hi,
The S041 is used for healthcare reasons only, and requesting this certificate will not affect the National Insurance contributions that you have already made in the UK.
However, the request for a S041 certificate would need be sent to HMRC direct from the authorities in the relevant country (Bulgaria in your case).
If the authorities refuse to request this certificate from HMRC, you can request a CA9110 instead.
You can request this by completing a CA3916 application -
Get a PDU1 certificate or a National Insurance contributions statement.
You must attach a covering letter to the application, stating that it is a CA9110 that you are requesting, and not a U1 or statement which is what that application is more commonly used for. With regards to your pension query, please contact the International Pension service on +44 101 2187777.
Lines are open from 8am to 6pm, Monday to Friday.
Thank you.