Safari "Stay on Page" pop-up keeps appearing

Every time I go to a website I get the following popup when I go to any link on the site:

It doesn't seem to matter what website, and I have to select 'Leave Page' to continue.

Generally this only happens the first time I navigate away from the original page.

Is there any way to disable or remove this?

I have tried zeroing all Safari caches and restarting machine, and staring Safari with shift key down, but nothing seems to make any difference.

Cheers

Mazcar