Thanks Jason - that's all fine now.
I'd used CSS on the contact form:
.searchbutton {
background-color:#FFF;
background-image:url(../images/search2.jpg);
background-repeat:no-repeat;
width:85px;
height:24px;
border:0px;
}
.searchbutton:hover {
background-image:url(../images/search1.jpg);
background-repeat:no-repeat;
}
But the search didn't like that, so had gone back to the image type.