OK thanks guys, have read the w3 schools stuff and I understand how z-index works now. But how I alter the z-index of a particular element is still a bit of a mystery to me.
I have set the page menu padding back to what it was originally, so you should be able to see the issue if you go back to the site now, although I obviously don't want to leave it like that for too long.
Thanks for your assistance so far.