It appears that since I applied for mr EIN to take advantage of the double taxation agreement with the USA the rules have been modified.
I discovered this when Kindle informed me that my EIN had not been accepted because of discrepancies. In fact this was probably due to the fact that I had not waited a few weeks for my new EIN to work its way officially into the IRS system.
At the same time it was brought to my intention that an EIN is no longer required. Instead you tick the box saying you do not have a USA TIN and then you are able to enter your UK National Insurance.
Instead of trying to figure out what went wrong with Kindle I used the UK NI route and it seems that I will not have USA withholding tax levied on US sales of my ebook.