Am I correct in assuming that at this point it is merely an insurance sign off process? - in other words anyone qualified on type can sign somebody else off as long as they fulfill recency / hours on type requirements mandated by the insurance company?
NO! As one or two have already pointed out there is no sign off required. As a non FI you are not qualified to sign off anything in a log book however; as a a pilot who is familiar with the type there is nothing to stop you familiarising another qualified pilot as part of an insurance requirement.
How you log it is another matter however; the requirements for logging are contained in Article 35 which states:
Particulars of each flight during which the holder of the log book acted either as a member of the flight crew of an aircraft or for the purpose of qualifying for the grantor renewal of a licence under this Order
Many will quote LASORS at you but that does not take into account all cases, and is primarily designed to ensure people have the correct hours for licence issue and revalidation. Beyond that, it doesn't matter what you log the flight as so long as you comply with Art 35. Many will state that you can only have one pilot in a single pilot aircraft; that in itself is not true. The law specifies the minimum number of pilots and the "operator" can decide how many are needed for a specific function. If you are checking someone out, you are clearly both acting as members of the flight crew and should log it as required in Art 35(2).