TKD wrote:
So, what happens if the Bus card-reader is broken, and the driver
waves you on - happens a couple of times per month on the routes that
I use (308, 678). Take it I should insist on paying cash?

You can add the 8, 277 & 15 to that list and countless more probably.

The official TfL position is that you should pay the cash fare and request
a refund of the price difference from them...

You can add 14, 430, 337, 74 & 85 to that list too.

So what happens in the cashless zone then? Oyster reader broken so you
have to get back off the bus, put money into the machine and get back
on... I can't see the driver waiting for you