Neil,
Yes there is a bush between the frame and brake backplate. 'Oily Bloke' asked the same question regarding his EW about 18 months ago. I posted a reply with photo and dimensions. The bush for your bike will be similar. If you are looking to make a new brake arm, the splines are the same as those in the drive gear for the pancake dynamo - I roughed out my splines, then used the hardened gear to shear out the last few thou. Dont try to remove too much metal this way or you will split the gear!
Regards,
Eddie.