domestic voltage stabilisers ?

I don't. I bought it in 2007 at Maplin (UNI-T UT58).

Cost £31. PP9 battery replaced in 2017.

How does one test these things, without access to a known accurate one ?.

Correct.

The efficiency of small motors is pretty poor, and so the magnetic and copper losses will be proportionate to supply voltage.

There are some inverter driven motors which I would expect to consume the same power independent of supply voltage. Inverter/smart fridge compressors are now becoming common.

I have a vague recollection of carrying out an experiment in my electrical machines course of applying varying frequency and reducing the voltage while measuring the current.

With induction motors the ratio v/f should be constant over a range of speeds when controlling output speed if I recall correctly from my electrical machines course some 50 years ago. To all intents and purposes UK mains frequency is constant so reduction or increase of voltage will affect output

My machines course was more recent :-)

Slip should be relatively small for an induction motor. Let us assume it might be 10% [1].

Slip is inversely proportional to voltage so a 10% drop in voltage would now mean rotation is now 89% of mains frequency (11% slip).

Iron losses are a very strong function of voltage, especially if the motor is designed for reduced cost.

I would expect iron losses to reduce more than the 1% loss in efficiency due to increased slip. BICBW