The main question to ask of the OP is why would he want to log it?
Is he planning to use this time for his 12 hours SEP currency to revalidate his licence? In which case, best not to log anything at all as if the CAA were to do a spot check and find no insurance he'd be in trouble.
If it's just for interest from a guy who has plenty of hours, just log it as passenger or P2 or something, making it clear it's not going towards any licencing requirement, and everyone is happy.