As already mentioned the voltage at the solenoid relies on the complete circuit having a good connection.
This is what I would do
1.Charge battery and confirm it able to hold voltage under load, and replacing it with a known good battery rules this out, or Get the battery tested
2. Check both battery terminals for a good connection, if Top Hat type replace for clamp type
3. Remove 5/16 half inch spanner size earth strap to boot floor clean both surfaces and replace, check for verdigre between cable and terminal
4. Move to engine bay and check the earth wire connection across the engine stabliser above the clutch end, or below on subframe to engine, If frayed replace, if ok check for verdigre between cable and terminal, Extra earth cables will help.
5.Remove the main battery cable from the solenoid (disconnect battery first) check and clean as required, clean starter to solenoid connections.
Check the crimped on connection on the solenoid and the solenoid to starter as they can be a source of high resistance with verdigre on the copper cable to terminal body
6. Clean the main Brown 12v feeds on the solenoid to the main power to fusebox and ignition.
7. Once all cleaned and confirmed no high resistance on any of those connection causing volt drop, reconnect battery and retest voltage across battery, and then repeat the test on the 12v onto the starter solenoid.
8. Thats the complete circuit
If your main battery cable has become damaged to be honest I've not seen one cause a 6v volt drop over its length, its usually a high resistance some where in the circuit and the tell tale is when lights slowly dim and fade away. The alternator wont have any bearing on any voltage checks on the circuit listed above.
If your familiar with volt drop testing and have a digital meter you could pinpoint where along the circuit is your problem, see FAQs Electrical on how to if your not up to speed on testing Volt Drop.
Edited by KernowCooper, 04 June 2018 - 11:58 PM.