Ive now tracked this down to /concrete/libraries/view.php

This section of code below $this->theme is looking for a theme called Peters note the capital P.
the theme directory is peters lowercase P.
If I rename the theme directory to Peters then the login and register pages work but the rest of the site fails. A temporary work around is to create 2 theme directories Peters and peters so the whole site works. is this a bug? I have created customised registration and login pages.

if (file_exists($this->theme)) {
include($this->theme);
} else {
throw new Exception(t('File %s not found. All themes need default.php and view.php files in them. Consult concrete5 documentation on how to create these files.', $this->theme));
}

Maybe removing and reinstalling the theme would solve the issue, after you make the theme directory name like the_theme_name using only small letters and underscores.

thanks for your suggestion. if i remove the theme will i lose any of my mods or will they load back in when i reinstate it?

If you changed page default, yes, they will be reset.
But the code will not be affected.

Make sure your site_them_paths are not set

$v->setThemeByPath('/login', "yourtheme");
$v->setThemeByPath('/page_forbidden', "yourtheme");
$v->setThemeByPath('/register', "yourtheme");
$v->setThemeByPath('/page_not_found', "yourtheme");