In some cases you would see the old 5A round-pin sockets being used for lights, such as stand lamps in hotels, etc. Using a 6A circuit for 13A sockets is wrong in a number of ways!I agree that 5 is odd but it is a 6A and IS controlling some downstairs skts. Will have another check tomorrow as to exactly what skts it is controlling.
As @westward10 posted it must be the Hager ones. Contrary to common sense and other industry norms, CU do not have standard busbars and so only the manufacturer's compatible ones should be used. This is an issue not just between manufacturers, but also in some cases between generations with the same manufacturer. Looking at you, Wylex!I do need to add another two cct breakers and will use RCBO's. Don't know what type/size to get though. I posted earlier the type that might be suitable. (Do they all tend to be mini ones?)
If you are contemplating changing the 16A MCB for a 20A RCBO you really must verify the cable is suitable, as whoever designed it originally used a 16A MCB for some reason. Quite what it was is not obvious, might have been a previous fixed load like an immersion heater, might have been due to them only having an old roll of 1.5mm in the van. Whatever the reason was is probably ost in the mists of time, so it becomes a job now to determin if the cable is safe for 20A in such a case.