I would assume you have a british version, which would be designed to run at 50 and not 60 Hz. If the motor is an AC synchronous (I assume it is) this would cause it to run approximately 20% faster than it should.
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Both these decks use shaded pole motors which are a form of asynchronous induction motor. Like all such, some of the power supplied is used to induce the magnetic field in the rotor.
The rotor field is induced by relative motion (slippage) between the rotor speed and the stator field speed. The stator field is synchronous with supply, so the motor must run at a lesser speed.
The greater the load on the motor the greater the slippage. The lower the voltage supplied, the greater the slip for a given load.
Altogether this means the motor speed depends on the supply frequency, the voltage and the load.
Exactly where in this continuum you end up is a matter of taste. For a long time I used 53Hz, 110 V and the brake fully on (mine is a 50Hz pulley) but I'm now playing with other parameters not available on a normal power supply so that will change.
My bad, I did mean asynchonous and not synchronous.
I suppose another option is to get ahold of a variable frequency drive. These are used to control the speed of pumps etc..., but if a small version could be found you could easily use it as your motor speed controller. Esssentially it allows you to control the voltage frequency, and hence the motor speed.
I (on occasion) have noted some of the belt sellers are supplying units with too large a diameter (belt), which will cause adjusting the platter speed to not be possible. Typically, platter speed that has been adjusted to its slowest possible setting will continue to be too fast.
If you have a 1/4" belt, replacing it with a correct (smaller) diameter OEM or [equal] correct diameter belt should return the pitch control to allow having a functional adjustment. A larger than correct size belt will ride on the outer portion of the pulley(s) [just] enough to increase pitch too much to be adjusted properly.