Statements of Truth

Wondering if anyone has applied for Probate on behalf of a client using the Statement of Truth rather than an Oath?

We have the Swat manual so have some guidance and a template there but not a lot of guidance on things like whether it can be used where one executor has power reserved.  Looked online and can't find much guidance on gov.uk. 

I have a feeling it's only currently available where all Executors are acting and should continue to use the Oath if one Executor has power reserved but would welcome any thoughts if others have knowledge or experience.

Many thanks

Lucy