Hello Stuart Hemming,
Only a limited list of tags and styles are supported by the Telerik Reporting HTMLTextBox item. Check out HtmlTextBox item help article, you will find the list of supported html tags and style attributes. As stated in the same article, the purpose of the HtmlTextBox item is to allow rendering of formatted text using HTML - it is not intended to cover the whole HTML/CSS specification. The item may be referred to as a TextBox with the capability to render differently formatted chunks of text.
Generally to print RadEditor content our suggestion is to utilize the RadEditor built-in print functionality.
the Telerik team