Assuming https://highnewportallotments.co.uk is the site with the issue, please install JCE MediaBox 2.0.14 Beta - https://www.joomlacontenteditor.net/downloads/mediabox/development

I have installed the new Beta version and it works on https://highnewportallotments.co.uk/

You do not appear to be using MediaBox on this site, rather it looks like you are using the Yootheme UIKit / WidgetKit lightbox?

Please send me a login -> https://www.joomlacontenteditor.net/contact/site-login

It looks like you've almost got this working, but there are some issues in your template's template.css file that are causing some display issues with MediaBox. In these css rules, on line 138+ of your template.css file, you should remove use of the !important statement, as this forces the style for all elements and cannot be overriden:

/* buttons */
.button, button, input[type="button"], input[type="submit"] {
	line-height:24px;
	color: #888;
	text-transform: uppercase; 
	border:0; 
	font-size: 11px !important;
	cursor:pointer;
	zoom: 1;
	*display: inline;
	vertical-align: top;
	outline: none;
	text-align: center;
	text-decoration: none;
	padding: 4px 15px !important;
	text-shadow: 0 1px 1px rgba(0,0,0, 0.1);
	-webkit-border-radius: 4px; border-radius: 4px;
	-webkit-box-shadow: 0 0 0 1px #BBBBBB inset;	
	-ms-box-shadow: 0 0 0 1px #bbbbbb inset;	
	box-shadow: 0 0 0 1px #BBBBBB inset;
	background: #f6f6f6;
	background-image: -webkit-linear-gradient(top,  #fff, #eee);
	background-image: -moz-linear-gradient(top, #fff, #eee);
	background-image: -ms-linear-gradient(top, #ffffff, #eeeeee);
	background-image: -o-linear-gradient(top, #fff, #eee);
}

.button:hover, button:hover, input[type="button"]:hover, input[type="submit"]:hover {
	color:#666 !important;
	cursor: pointer; text-decoration: none !important;
	background: #eee;
	background-image: -webkit-linear-gradient(top,  #fff,  #ddd);
	background-image: -moz-linear-gradient(top,  #fff,  #ddd);
	background-image: -ms-linear-gradient(top,  #fff,  #ddd);
	background-image: -o-linear-gradient(top,  #fff,  #ddd);
}

I will have go this evening removing those css files that are causing the issies

Not the css files, just the !important statement in those particular css rules I highlighted above.

Yes.