"""Cable length is slightly under .3 metres total round trip. This gives 0.21 volts drop at 130 amps using Ancor's published resistance of the cable. Also, Lofrans reckons #2 wire is OK for up to 25 metres round trip

Agree the drop is large. I will dig into the battery and measure voltage at its terminals and check the other things mentioned

Since my first post, we measured current drawn at no-load, with chain off gypsy. It was 180-200 amps, with windlass turning at 40 rpm, which corresponds well with the advertised maximum line speed of 40 m/minute ""

Voltage drop is large but not unexpected. Lets say you have 4 volts drop ( 8 to 9 volt at the motor )when running at 130 amps, that is only .03 ohms. Between wire and inter battey resistance easy to see .03.
No load It was " 180-200 amps", I hope someone miss read the meter. Maybe 18 to 20 amp??? I had a winch fail when the magnets became unglued. No power and high current draw. This was after 10 years in the hot sun of Baja. In a new windlass, one of the magnets could have been put in backwards. I would guess bad motor.