SOLVED.
The problem is with Position Fixed DIVs with a specified width in pixels.
If the viewport is being controlled by media queries (specifically for WinPhone 7/8) then specifying a width greater then the screen width in a fixed div causes the problem.
Here is the start of my altered CSS - note the div.header and div.site entries. NO SPECIFIED WIDTH - just 100% inherited from body with a max-width thrown in.
body
{width:100%;
max-width:768px;
min-height:1028px;
margin-left:auto;
margin-right:auto;
background-color: #ffffff;}
@font-face {font-family: HelveticaNeue;
src:url(fonts/HelveticaNeueLTStd-Lt.otf);}
@font-face {font-family: FuturaStd;
src:url(fonts/FuturaStd-Book.otf);}
div, span
{font-family: HelveticaNeue, Arial, sans-serif;
font-size:120%;
font-weight:normal;
text-align:justify;
color:#202020;}
div.site
{max-width:768px;
min-height:1028px;
margin-left:auto;
margin-right:auto;}
div.header
{position:fixed;
z-index:5;}
And Here are the media and viewport elements I used.
@media screen and (max-device-width: 25em)
{body
{-webkit-text-size-adjust: none;
-moz-text-size-adjust: none;
-ms-text-size-adjust: none;
-o-text-size-adjust: none;
-text-size-adjust: none;}}
@-webkit-viewport{width:device-width}
@-moz-viewport{width:device-width}
@-ms-viewport{width:device-width}
@-o-viewport{width:device-width}
@viewport{width:device-width}
And here is the head viewport meta tag in the html
<meta name="viewport" content = "width=device-width, maximum-scale=1.0"/>
Hope this helps.
T.