Why Does My Nottingham Motor Seem To Run Fast?

Glad you got it resolved.
Yes if riding out of the groove it's like using the wrong diameter pulley.

I believe Hollywood Sound in Florida is still the official dealer for Nottingham.
You should be able to order the correct belt from them.

Good luck!

Ned, In your first post, you wrote, "Not impressed with the motor either, unless I spin it really fast it never gets up to full speed."  I am not sure what that meant, but I assume whatever the problem was, it has also been cured.  Keep in mind that the NAS turntables were deliberately designed to use a weak motor and a heavy platter.  The idea is that the motor only is there to get the platter up to speed whereupon it maintains speed mostly due to its rotational inertia.  The motor is only to give it a little oomph, once up to speed.  I no longer own a Notts tt, but when I did I found that a Walker Audio Precision Motor Controller was a major upgrade, like getting a whole new turntable up another level in SQ.

Been in touch with Notts. Their belts are 3mm, the one I got was 4mm.

As Lewm stated
Notts platters/motors are relatively high mass/low torque affairs.
In fact the Notts motor is designed to be left turned on at all times and it just sits there until you give it a push start which does require a healthy spin .
And then to stop you actually catch the edge of the platter with your hand to slow it down and stop it.
Sounds archaic but it works fine and they usually hold good speed regulation and even better with a Walker speed controller.
I bought Lewm old Walker from him and I was very impressed with the difference it made.
Something to consider if you plan on keeping the Notts. 

I am enjoying its sound but will refrain from judging it properly until it has got a proper belt and had an oil change. Currently having to use my old faithful project tt as it is more dependable and isn't currently using cotton thread as a belt. May post an update once I get it maintained.