I think Pauluk got it when he showed the comparison between cable sizes, US commercial grade cords are *much* heavier than the UK equivalent.
A pro-grade 2.5mm cord reel (2.5mm being approximately 13 Guage) would be fine wound up even at 13 amps I would think, maybe even 16.
This cable, apparently wasn't.
I have certainly seen grossly undersized cords used coiled, for long periods, with no apparent ill effect, and then something like this comes along and makes you go 'hmm'.
I don't think 'magical' superheating is likely at all, but it may make something marginal, go just the other side of that margin.
