[Nonfractal]

Jeremy,
Sorry it seems you didn't have a perfect Christmas day.
No one likes it when their new toys don't work.

I've searched lots of sources to definitively find out about polarity of scalextric but terminology messes up any concrete findings.

Eg  "common ground" is not the same as "negative polarity" 

To do some fault finding, unplug from the scalextric power base and rig your SCP2 up like this (pic directly from the SCP2 PDF)
You can use a pp3 9v battery. 

If your test car works like this, its a problem with the scalextric power base. 
Maybe  a fault, maybe polarity. 

Hope this helps. 
Alan W