The issue was caused by the length of my HDMI cable which was 3M 1.4a HDMI spec.
It is because the default power output on the HDMI connector is sometimes not enough for longer cables causing the zits, in order to fix this you should edit your config.txt file in the root of your SD card (will be visible under windows explorer, if the text file does not exist then you can add the file) and add the following line:
You may need to increase or decrease the value in range 1 to 7, where higher numbers are for longer cables.
For me the above worked and fixed the issue.
There is further information here