Aerial hole size

I have a 52 plate Fabia Mk1 Estate without roof rails but with a standard Skoda aerial fitted.  Most of my music is currently streamed from my mobile as I live in area with very poor radio reception and I'm in the middle of modifying the standard fit MS402 as detailed at the following website http://hacks.ostserver.co.uk/2012/01/car-audio-skoda-ms402-aux-in-retrofit.html

The only difference is I'm doing away with the Aux socket and wiring in a Bluetooth receiver instead.  My plan is to remove the standard fit antenna base and fit a base for my amateur radio antenna instead.  This base requires a hole of 13mm diameter.  Does anyone know the size of the standard hole that has been drilled for the original radio antenna?  I want to check before removing it because if it needs enlarging and may look at different options if that is the case.  Currently it is on a boot mount but I'm getting a bit fed up of trapping the cable in the boot hinge every time I shut the boot.  Lets just say I've had to repair it a few times already!!

Any help is appreciated.

Ryan