Applying for an S1 with the new A1 process

Now there is a new process for the A1 application that includes possible entitlement to an S1, should a UK residual address be entered or their foreign residential address (as this is what used to be used for eligibility of an S1), as there is not an option for both? Additionally, those who have been posted to a country just before the system changed can no longer apply for an S1 separate, what is the best way to ensure they receive an S1? Is it reapply for a new A1 and partnered S1, or is there a better way?

Hi,
Our processes have recently changed, and we now require you apply for the A1 in order to also receive the S1.  
The application form you will need to complete is the CA8421 Apply for a certificate to confirm you pay UK National Insurance when working in 2 or more countries (CA8421).  
It will ask a number of questions to determine your circumstances, one of the questions asks if you only work in the UK, and an A1 and S1 will be issued if you are eligible.
Any additional information can be added in a covering letter.
Thank you.