On 4 Feb, 12:57, NT wrote:

Years ago we used to run 15A continuous loads on standard 13A plugs.

I've done loads of this over the years, chiefly welders, and had
little trouble (but as you say, use MK kit). Until you try to unplug
it, then you find the contacts are welded in place...

On the whole, I wouldn't recommend it. The worst risk is probably
chronic overheating, then the insulator failing and live contacts
dropping into contact with the earth. If you're not RCDing because
it's a leaky or inductive load, and it's it's fused at mongo current
anyway, then there's far too much risk of a firestarter arc developing
and neither safety provision tripping. That's how Americans burn their
workshops down.