[NewfieDrool]

Try this first.
On MacBook open up system preferences.
Click on network
Make sure on the right you are on the setting you want ie.WI-FI
You will see a button on the lower right called Advanced, click this.
You will then see all the networks available.
You can drag the preferred network to the top of the list, highlight and drag.

This should force your MacBook to pick the network as default.

Edit, just seen you did this and also deleted leaving the preferred network. Should work as that is all I have done and it's fine on my MBPS.

I did enter a channel by manual means and I selected channel 48 but I did my friends network the other day and she has a TC and an Extreme as an extender and again I had no problem forcing her MBPS to the 5ghz channel.