Hi tdevip,
If you will not be taking any salary payments or providing any reportable benefits or expenses, then you will not be required to register for PAYE as an employer.
The SIPP is a personal pension and is not classed as a workplace pension.
An employer can make contributions to a workplace pension and guidance on this can be found here:
Workplace pensions
If you need further help in relation to SIPPs, you can contact MoneyHelper on 08000113797.
You can also find further guidance on how SIPPs work here:
Personal pensions.
Thank you.