It was the host like you said. I had to create a php5.ini file with the contents:
display_errors = on
error_reporting = E_ALL

So now I know the error is:

Fatal error: Class 'ImageGallery_Controller' not found in /home/sites/mysite.co.uk/public_html/mysite/code/MyImageGalleryPage.php on line 6

Do you know how I fix this please?

BINGO! all works like a charm, thanks so much for your help UncleCheese!!! wicked.

I'd didn't realise it would create a brand new page type (noob error I expect) so it just means I have to re-build the gallery page but thats fine! all the new options are in the pop-up box and they show in the live site, just need to add my red dot images etc.
I will post the link to the final site when its finished as an example of it in action. Thanks again!

Could the drag and drop not be working in the new pages MyImageGalleryPage because of this that I read in the docs?:

"Activate Drag-and-Drop Sorting

Because drag-and-drop sorting adds custom functionality to all eligible DataObject subclasses, the DataObjectManager requires that the user opt into sorting capability.

mysite/_config.php

SortableDataObject::add_sortable_class('Testimonial');

After running a /dev/build, the following will be modified to give Testimonials sorting capability.

*
The field SortOrder will be added to the Testimonial table

*
The $default_sort property of the Testimonial will be set to SortOrder. This will ensure that the frontend template displays the objects in the correct order.

Note: To add several sortable classes at once, use SortableDataObject::add_sortable_classes(array $classNames)
"