Thanks to everyone for the additional information. The replacement fittings I am putting up are fluorescent striplights with large cable entries and chunky terminal block for the wires so 4mm² won't be a problem.
The lighting circuits in my house dated from the days before earths were common on such things. Most of it has been replaced but the lighting spur to the garage still has no CPC in the cable.
I would like to replace the existing pendant bulbs with fluorescent fittings, 2 of which I have for the...