Thanks for your responses. My gallery is at
http://www.enfieldsociety.org.uk/photographs/The problem occurs in all themes, including Curve. The source of the displayed home page starts like this:
keyword<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en" dir="ltr">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta http-equiv="Pragma" content="no-cache" />
<title>Home - TES photographs</title>
<link rel="stylesheet" href="css/coppermine.css" type="text/css" />
<link rel="stylesheet" href="themes/tes/style.css" type="text/css" />
You will see that the text "keyword" appears at the very start, before the doctype declaration. It also occurs in help boxes such as the one shown in the first attachment. The source text for that box starts like this, again showing "keyword" at the very start:
keyword<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en" dir="ltr">
<head>
<title>Help</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<link rel="stylesheet" href="css/coppermine.css" type="text/css" />
<link rel="stylesheet" href="themes/tes/style.css" type="text/css" />
<script src="docs/js/jquery.js" type="text/javascript"></script>
<script src="docs/js/help.js" type="text/javascript"></script>
</head>
<body class="nobgimage">
<div id="cpg_main_block">
Curiously this specifies <body class="nobgimage">, but that definition occurs only in the style.css of the Curve theme, which is not being used. I tried adding it to the style.css of my own theme, "tes", but it made no difference.
I have spent a day checking through all the files to look for a spurious "keyword" text, but have not found any. I reloaded the core files, as unzipped from the dowload file cpg1.5.46.zip and have not modified them. Version check, however, shows many of them as "modified" - see the second attachment. The file sizes on the server are slightly less than on my Windows 10 machine, but I presume that that is just due to the format in which they are held - comparing the content of sample files shows no difference. I have not created anything in "anycontent.php".
Any more help or suggestions would be most appreciated.