Unwanted pop ups (lp.jzip.com) on every click

Everytime I click on a link on my site I get a pop up ad with a url lp.jzip.com.
I have tried it on another computer and it also happens there.
It doesn’t happen with other sites I visit (using IE10).
It still happens when I deactivate the plug ins.

My site is causeaction.co.uk

I obviously have a bug of some kind – any help to remove/resolve it will be much appreciated. Thanks in advance.