.....I would have thought provision would have been made by the manufacturer to have such a heater in constant use.
It depends on the instruction manual with the device, they almost all give instructions "suitable for a normal room up to 6m^2" or something similar.
This isn't just buff, it's limiting the use to a place where it can maintain the typical temperature whilst remaining within the duty cycle it's designed for!
If you want a device that may be operated continuously, then you need to buy a suitable device.