if you can enter on the line do you have to be an audax member beforehand?
No, but you have to pay a supplement for 3rd party insurance on the day (£2?) unless you can prove that you already have it (e.g. with CTC).
These Audax thingies-what's the etiquette at stopping at cafes en-route?
Do they usually pass cyclist friendly cafes or are you expected to sort yourself out?
Audax is about being self-reliant so stop where and when you want. The control points are often at cafes, but between them, do your own thing. Obviously if you are riding in a group, as some of us intend to do on the MNWP, then discuss it among yourselves.
I think that it is extremely unlikely that an event organiser would forget the need for riders to pick up food and drink on a ride, so (s)he would take that into account when planning the route.
The etiquette question comes in at control cafes, where you are encouraged to buy something as a way of saying thanks to the cafe owner for use of their facilities. You don't have to buy an expensive meal, even a can of Coke or a Mars Bar would do.
The only one I have done SITD, one of the control points was at a cafe so we stopped there.
Was also a control point in a layby where snacks/bananas/water was available free.
SITD and its sister event SoM are at the lavish end of the scale in terms of catering! The MNWP is more basic - control at Country Kitchen cafe at Waddington, pie-and-peas at the start/finish pub!
It is 120 km so 1 stop should be okay, whereas on a 200, there would probably be at least 2, maybe more.
My forum ride on Sunday took us to Country Kitchen. I saw that they had a thank-you letter from Audax UK pinned to their noticeboard, and also a letter from Chris Crossland, organiser of SITD and SoM.