Does this mean, for example, someone getting on a Routemaster, which has a
conductor, will be unable to buy the ticket from the counductor when in the
"ticketless" zone, but can buy one from him when the same bus is outside that
zone?

If so, is it actually going to be the case that a passenger trying to buy a
ticket from the conductor in that zone will be put off the bus, or told "wait
till we are out of the zone and I'll sell you a ticket then", or will
commonsense prevail and he will sell you a ticket as usual?

Marc.