Jimbo....are your earth straps / positive leads all in good order, clean etc, and the earthing points clean and 'solid'? Often the ends of these leads are factory crimped to the copper core, and in time dirt & vibration can penetrate / destroy the continuity needed for these high currents.
A 3V drop appears, if anything, a bit low, implying a lesser current drain than might be expected for cranking.
If all connections / leads pukka then perhaps a dirty starter solenoid - easily checked by using a car jumper lead from the starter motor live direct to battery +ve. But... take care, biggum sparks.... don't do this anywhere near petrol etc.
If still sluggish try the same jumper lead trick, but now starter motor casing direct to -ve of the battery, but this time use the starter button.
Apologies if all the above 'obvious' & already tried.
Cheers.