Finally received my diploma from Zend today. I passed the exam way back in November last year and had emailed Zend twice asking politely, when I would be receiving it, but never heard back from them.
It appears to have taken the best part of nine weeks to get here, for some reason they've marked it 'Surface Transportation Only.'
The diploma came in a little frame and I got a couple of stickers to go with it. Apparently I can claim some free ZCE business cards from some online company aswell.