Mate! I have the same problem.
I am still working on it, but all roads lead me to the alternator (plus the terminals)
I'll explain.
1. Get your hands on a voltmeter... no way to help this without one
2. Put the voltmeter on the alternator (on the pins) (ie/ start the car, then put probes on the alternator terminals. How many volts?
3. Put the probes on the solenoid +ve (and an earth somewhere - like engine) How many volts?
4. Put the probes on the battery terminals - How many volts?
This is best done with a friend, get them to rev it each time and get idle reading and revved reading.
Also, do you still have the Lucas Fusebox? with 2 feeds and about 9 out? What I found was my fusebox connections were so crappy that I had power on one side, and hardly anything on the other. I replaced the fusebox and miraculously many things began to work.
The charging discharging problem I have still is not fixed. I took it to a sparky (to double check myself) and he said the Alternator isnt charging... oh, but now it is... oh there it goes again... hmmm (*sparky scratches head and says 'FU$%ING LUCAS) I am NOT joking... I fell over laughing.
One last test if your friend is happy to help is get the voltmeter on the battery terminals and try to kick the car over... how far does the voltage drop? I go from 12v to about 3v ... my battery might need replacing too.
Is there water (liquid) in the cells of the battery?
All things to check
Peter.
PS: If you do by a new battery get one with BIG CCA (just ask the battery place or sparky you need lotsa CCA (Cold Cranking Amps)