Symptoms...

Battery dead, replaced with new one and  it lasts about a week before flattening again. Yet, subsequent charging may last a day or two or again another week...is it an intermittent fault?


You might have something draining the battery intermittently. It is unlikely that a faulty earth would cause this to happen. There may also be an intermittent fault in the charging circuit, which could be a poor earth, as David has said.
 
You can check the earths yourself. Follow the black wires from the battery onto the wing and check they aren't corroded where they bolt to the chassis. Find the earth strap that goes from the body to the engine near the gearbox and make sure that it's not corroded at any point.
 
If you are unsure of electrics you may need an auto electrician to check the current drain from the battery, and the charging voltage, although you could buy a cheap multimeter and do it yourself. It needs to have a DC current range of 10A. Disconnect the battery positive and connect the meter on its 10A range inbetween the battery positive terminal and the red wires. Make sure the ignition is off, and do not turn it on, because it may damage your meter. The alarm will draw a small current, maybe a tenth of an amp, possibly much less. If there is more than that, you need to start pulling fuses to isolate the faulty item.
 
To check the alternator, refit the red wires to the positive terminal and start the engine. With the meter set to DC volts (and check if you need to move the red plug on the meter as some meters use a special socket for Amps), connect it across the battery (red to red, black to black). The meter should read around 14 volts. Rev the engine a bit, and it should not exceed about 16 volts. If it reads less than 13 volts at any time you have a poorly charging system.
 
Hope that helps, but any auto electrician can do those tests for you very quickly if you are unsure of what to do.
 
Jon