Thanks both,

the positive side of the alternator circuit is fine ~0.3 voltage drop to battery positive terminal. I got zero voltage drop between the alternator casing and the negative battery terminal which seems unlikely - maybe it is mounted on rubber bushes with a separate earth strap (I'll check).

Alternator output with everything switched on is 14.5 volts though it does drop at full lock when held against the steering stops to 13.8 volts - significant? The loss of assistance can happen anywhere in the steering range not just at full lock.

The connections to the motor/pump need the front bumper off so I'm waiting for better weather.