This was a new one to me, I couldn't see a single thing wrong with my template, the right Freeform ID and name, all the tags matching up.  The form just didn't submit.  Looking at the network tab I could see the POST with a 200 (not the expected 302) and I could see all of the input fields in the request.
{exp:freeform_next:form id="enquiry"  class="mx-auto"  form_name="enquiry"  use_required_attribute="yes" form_attributes:accept-charset="utf-8"}

So what was going on?

This is what happens when you are changing things on the hop.  I had changed my mind about the fields to include in this form.  Too many fields and the visitor may be put off, too few and the message might be just too vague.

Essentially I had removed one of the fields from my template but I hadn't removed it from Freeform. 

It fails silently.

I thought that I would share this as it is such a silly mistake but soaked up far too much time!