Custom Menus module causing a logout mis-function

Hi there. I'm fairly new to Gallery, but so far I have managed to solve all my little problems with a bit of searching and general IT knowledge.

I am now stuck on a small problem. I want to add an 'escape' button so that viewers can get back to the page they came from on my site. I downloaded and installed the Module:Custom Menus by rWatcher, activated it and created a new button 'Return to Site' with / as the url (to go to the default index page on the site root directory). That bit works very well.

My problem is that when I 'logout' from admin the url is to a non-existent page. It should go to (for example):

pics.mysite.org.uk/Gallery/index.php

instead the browser shows it's trying to find:

pics.mysite.org.uk/Gallery/http%3A%2F%2Fsite.mysite.org.uk%2FGallery%2Findex.php%2F

Going back to Gallery shows that I have logged out ok, and everything else works. Disabling the module means I lose the new button, but the 'logout' behaves correctly. I have seen that others have used the module, but no-one else is reporting this issue, so I'm guessing there may be something in my set-up that is wrong. Have I missed something?