So it appears on several sites that TextFX's HTML tidy is both excellent and has also not been supported since 2008.
It appears that the latest version of Notepad++ completely breaks it in fact.
Now I've got some HTML on a single line that I need to indent, etc. any replacements for HTML Tidy on Notepad++ that anyone can recommend?
I use NppTidy2 plugin, it works pretty well.