I haven't tried this but try adding z-index:99999 to an li CSS style for the sub-menus (the ones that fly out). 
Thanks John but I think that went over my head.
I have created the menu using WebAssist Menu Writer, I'm not sure where to add what you have suggested, I have the following files.
menu.ssi which as the extension suggests, is an include.
Then the folder automatically created by WebAssist which contains the following files:
menu.css
menu.js
menu_ie.css
menustyle.xml
treexml.xml
I guess it must be both css's but where, once again I guess at the '-1level'
I've had a play, but no success yet.
The code for 'menu.css' is as follows:
ul#cssmw {
cursor: default;
list-style-type: none;
margin: 0 0 0 0;
padding: 0 0 0 0;
width: 0;
}
ul#cssmw ul {
cursor: default;
font-size: 0;
list-style-type: none;
margin: 0 0 0 0;
padding: 0 0 0 0;
}
ul#cssmw li {
background-image: none;
padding: 0 0 0 0;
position: relative;
white-space: nowrap;
}
ul#cssmw li ul {
display: none;
top: 0;
}
ul#cssmw li:hover > ul {
display: block;
position: absolute;
}
ul#cssmw > li {
background-image: none;
}
ul#cssmw > li > a {
background-image: url("../cssmw_images/bg.jpg");
background-repeat: repeat-y;
border-bottom: solid 1px #9BB6D2;
color: #FFFFFF;
display: block;
font-family: Arial, Helvetica, sans-serif;
font-size: 12px;
font-style: normal;
height: 14px;
line-height: 14px;
margin: 0 0 0 0;
padding: 6px 10px 6px 6px;
text-decoration: none;
width: 134px;
}
ul#cssmw > li:hover > a {
background-image: url("../cssmw_images/grad.jpg");
background-repeat: repeat-y;
border-bottom: solid 1px #9BB6D2;
color: #0D092B;
text-decoration: none;
}
ul#cssmw ul.level-1 {
left: 150px;
}
ul#cssmw ul.level-1 > li {
background-image: none;
}
ul#cssmw ul.level-1 > li > a {
background-image: url("../cssmw_images/bg.jpg");
background-repeat: repeat-y;
border-bottom: solid 1px #9BB6D2;
color: #FFFFFF;
display: block;
font-family: Arial, Helvetica, sans-serif;
font-size: 12px;
font-style: normal;
height: 14px;
line-height: 14px;
margin: 0 0 0 0;
padding: 6px 10px 6px 6px;
text-decoration: none;
width: 134px;
}
ul#cssmw ul.level-1 > li:hover > a {
background-image: url("../cssmw_images/grad.jpg");
background-repeat: repeat-y;
border-bottom: solid 1px #9BB6D2;
color: #000A5C;
text-decoration: none;
}
ul#cssmw ul.level-2 {
left: 150px;
}
ul#cssmw ul.level-2 > li {
background-image: none;
}
ul#cssmw ul.level-2 > li > a {
background-image: url("../cssmw_images/bg.jpg");
background-repeat: repeat-y;
border-bottom: solid 1px #9BB6D2;
color: #FFFFFF;
display: block;
font-family: Arial, Helvetica, sans-serif;
font-size: 12px;
font-style: normal;
height: 14px;
line-height: 14px;
margin: 0 0 0 0;
padding: 6px 10px 6px 6px;
text-decoration: none;
width: 134px;
}
ul#cssmw ul.level-2 > li:hover > a {
background-image: url("../cssmw_images/grad.jpg");
background-repeat: repeat-y;
border-bottom: solid 1px #9BB6D2;
color: #000A5C;
text-decoration: none;
}
ul#cssmw ul.level-3 {
left: 150px;
}
ul#cssmw ul.level-3 > li {
background-image: none;
}
ul#cssmw ul.level-3 > li > a {
background-image: url("../cssmw_images/bg.jpg");
background-repeat: repeat-y;
border-bottom: solid 1px #9BB6D2;
color: #FFFFFF;
display: block;
font-family: Arial, Helvetica, sans-serif;
font-size: 12px;
font-style: normal;
height: 14px;
line-height: 14px;
margin: 0 0 0 0;
padding: 6px 10px 6px 6px;
text-decoration: none;
width: 134px;
}
ul#cssmw ul.level-3 > li:hover > a {
background-image: url("../cssmw_images/grad.jpg");
background-repeat: repeat-y;
border-bottom: solid 1px #9BB6D2;
color: #000A5C;
text-decoration: none;
}
