...one of those quasi-legal check flights where nobody's quite sure who the PIC is...
There is no such thing. Either you regain recency by flying solo or you do so with qualified instructor.
However, if you mean 'receiving variant familiarisation' on an SEP class variant which is new to you, that does not require an instructor, it seems.
A certain 'minister-without-briefcase' is trying to persuade the CAA to change the 90 day recency requirement for non-EASA aircraft, but has few supporters outside La-la Land.