Was wandering round local hardware shop while Mrs Robo was getting some keys cut and had a look at the electrical section. Looked at the helpful labeling like 2.5mm² T&E "for use on ring mains" etc., when I saw some red & black singles marked "not for mains voltage" had a look at the side of the reels, it was 6491X, looks like I've been using the wrong cable for the last 35 years