OK, not sure if using a Ditto call on the current document is the right approach but everything now works by using &documents=`[*id*]` to reference the current document.
In the template for the blog post page I have the following Ditto call:
[!Ditto? &documents=`[*id*]` &tagData=`tags` &tagDocumentID=`55` &tpl=`newsPageTags`!]
The only code in the &tpl referenced in the above call is:
The &tagDocumentID=`55` in the call references the document that lists my tags as described in the tagging video here:
http://ditto.modxcms.com/screencast/tagging.html
Sorry for talking to myself but I like to make a note of this stuff so I can find it easily later!
Please comment if there is a better way of doing this, as I say, using a Ditto call on the current document seems like a bit of a hack to me.
Cheers