Hey Nigel,
I believe you are using the new "Composer Editor" to compose your blog posts and up to 5.0.15, we are automating the truncation of blog posts by blocks rather than characters or paragraphs as opposed to the WYSIWYG truncation settings.
I have updated this behavior internally and in 5.0.16 which will most likely be released tomorrow or in a day will have these automated truncation by characters for posts created with the new composer.
Temporarily, what you want to do is to split your text by blocks. For instance, if the settings under Settings > Layout > Truncation (The section on the left that says truncate by number of blocks), is set to 1, you only need to split your text to multiple blocks in the composer.