Are you sure the extensions have been wired as "external" extensions? I have just had a look at the appropriate N diagram (N4507 - page 3) & according to that the only thing connected to "Block Terminal No 37" terminal 13 should be a connection to terminal 1 of "Convertor Ringing 9A".
As Tony has said you are best off wiring these as 3-wire extensions. (I have done this for a set I have created & can provide details if required)
I have attached the appropriate N diagrams which I obtained from Sam Hallas's excellent site -
http://www.samhallas.co.uk/repositor...rams/index.htm
You can usually ascertain which version of the Planset you have by looking at the base plate of the Planset. It usually says whether it's a mark 1, 2 or 3. Failing that you can take off the base plate & comparing the circuit layout with the pictures on diagram N625. If you do have a mark 1 be careful that the power supply or connected with the correct polarity as if not I believe it blows the 2 transistors in the Planset base.
Also reading you note about only red & white being used in the cable to the main. Do you mean the cable from the terminal block to the Planset base? This should be quite thick & have 18 wires in it.
If it helps post some pictures of the connections & we will take a look & advise.
Regards,
Wayne