Paddy, I have the chinook pin base
http://www.chinooksailing.com/products/product_info.php?products_id=110and the aeron skinny alloy extension.
www.aeronsport.com/Web/index.htmlAll good.
Secondhand look for wear in the fixings (pin or cup) that hold the base into the extension and that the pulleys still rotate smoothly. Make sure you change the rope. Formuline is expensive but makes downhauling a breeze.
A new base is a good investment. But if you do go secondhand look for (looseness) wear and replace the tendon or joint.
The chinook tendons (holes at 90 to each other 31.95) are more expensive than others at $20.
I have a couple of spare chinook cup double bolt base plates if you need em.