Its not a bad idea as long as the ticket machines work and twice now I've
been unable to get a ticket from a machine because it either wasn't
working
or had been vandalised. Both times I had to argue with the driver and
eventually he let me on but basically if the machine is buggered you
cannot,
according to the rules, get on a bus.


No, according to the 'rules', if the machine is broken the bus driver must
allow you to board and proceed to take you to the next working machine.
While you're buying a ticket at this new machine he must wait for you. If
however you simply forgot to buy a ticket in the cashless zone and you want
to board the bus, the driver will tell you that you need to buy a ticket
before you board and is not obliged to wait.