The Lucas ACR alternator, like many alternators fitted to British cars, has the voltage regulator mounted inside. This is possible because the modern voltage regulator is a sold-state device.
When you have tested the charging system of a car fitted with an alternator, and the checks in How to test a car battery point to a fault in the voltage regulator, make sure that you need to replace it. The fault may be elsewhere.
If the simple tests described here do not work, take the car to an auto-electrician; alternators fitted to modern cars are easily damaged.
Before doing any work on an alternator system other than testing, disconnect both terminals of the battery.
Incorrect charging or no output may be due to a badly earthed regulator. Make sure that the connections are clean and tight. The unit may be earthed through its mountings or by a separate lead.
Undercharging may be caused by faulty alternator brushes and slip rings (See Renewing alternator brushes), as well as by a faulty regulator.
A simple way to check the alternator is to start the engine and connect a voltmeter across the battery terminals. If it registers battery voltage only, the fault is in the alternator or its wiring, or in a field isolating relay if fitted.
If it registers an excessive charge (15 volts or more) the regulator is faulty and you should replace it.