Thanks for the replies, very helpful.

Just to add to Dan's comments....

There is nothing magic about an exact figure like 14.5V.  A fully charged battery in good shape will read about 12.5V at rest.  You cannot measure that right after turning the engine off, you need to let the battery sit for a while.  You can either wait for a while or turn on a load (like the headlights) for a minute, then turn them off.  What you want to see is that when the alternator is running the voltage across the battery terminals increases by 1 to 1.5 volts from what you measured at rest.  Typically you don't want the charging voltage going much above 15V or you risk boiling away the electrolyte in the battery.  You don't want to see less than a 1V increase because that calls into question whether the battery can/will recharge.  Fluctuations in the charging voltage are to be expected as the electrical load of the car changes.  

No, I wouldn't want to see the voltage bounce around rapidly, but I would expect and be comfortable with the voltage drifting around a bit.  Another alternator test is to take your multimeter and set it to AC volts instead of DC.  Connect it across the output of the alternator and ground.  Monitor that for a little bit while you drive.  You will see some AC ripple but typically you don't want to see more than 100 mV AC.  The higher the AC ripple, the more chance you have something wrong with the regulator circuit.

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An alternator will produce charging voltage if it is turning fast enough. If idle drops too low or the belt is loose or some other cause, it won't produce enough voltage to overcome the battery's actual voltage - nominally around 13.2 volts (6 cells @ 2.2v). Low enough and the charge light will come on. With the engine off(and everything else) and the key on, your voltmeter should be seeing the battery voltage available at the meter. A built in meter may see a little less due to resistance, weak connections etc. in the electrical system.
Voltage regulation might better be described as voltage limitation. That's what the mechanical regulators do. The basic reason is to prevent too much voltage going to the battery and cooking it. The common symptom of that is a rotten egg, sulphurous smell while the battery is being over- charged. On cars with the battery up front, your nose can usually detect it. In a Mini, with the battery in the boot, it is harder to detect. Look for a warm battery. With newer, 'sealed' lead-acid batteries, it may be even harder to spot, though those batteries are vented.

There isn't much difference between 14.5V and 'below 15V'.

Alternator voltage, as shown on the cockpit gauge, varies from just above 13 volts at idle to just below 15 volts at cruise.  I thought that alternator voltage was supposed to remain a fairly constant 14.3-14.5 volts, regardless of engine speed.  Yes or no?