Righto, so this is the fix ( although I may be counting my chickens!! );
because we are using Sendgrid, due to the fact we have a database of 4k users already, the Sendgrid system was being blocked by gmail before entering the gmail servers:
Site -> SendGrid ->Gmail-> User
X-----breakage happens here
Sendgrid doesn't report on Gmail blocking, but a few others do, so I was able to understand the mechanics involved; SG use numerous SHARED servers to handle the distribution of the load of emails going through their system from your site and all their other clients. 'Other clients' is the clincher; it takes very little before one of the many shared servers gets marked as a spam source, which takes a really long time to clear. And so because your neighbour was naughty you get tarred with the same brush.
The solution is paying for the rather expensive "dedicated IP" of the Pro account, so that you are only ever using one, dedicated IP address to send your traffic, rather than the many that would otherwise occur in Shared space.
eg Shared traffic is split over Shared Servers:
and Gmail blocks ....212 because some spammy stuff went across it last week, thereby reducing your deliverability to Gmail by 25%.
Change to Dedicated IP:
And Gmail lets everything through, because you're not sending spam, right? ;-)
Mailchimp does the same thing, apparently, but at a higher cost.