The manpower needed to produce Warspite's steel

How long would eight people take to mine and smelt the 31,000 tons of steel specified for Warspite?

Years upon years if they worked alone, with no precise estimate given; recruiting more workers would change the timescale.

Eight people working by themselves would need years upon years to mine and smelt the 31,000 tons specified in the question. Recruiting a larger workforce would change the timescale substantially. No production rate or definite completion time is calculated, so the answer remains a qualitative judgment about the labour needed.