Hi Gareth
Based on a quick internet search it seems that:
"On November 6th 1947, a miner was crushed by a collapsing wall at Marine Colliery, Cwm, Blaenau Gwent."
Fairly sure that the accident was fatal, so Mrs Edwards probably his widow.
The circumstances leading up to the later judgment (1954) in the House of Lords in Marshall v Gotham were apparently similar, i.e. latent weakness in the structural stability of an underground mine.
The judgment in Marshall v Gotham makes reference to that in Edwards v NCB but chose to dispense with the word "grossly" from its interpretation of what "reasonably practicable" means.
Hence leaving it to a defendant in either criminal or civil action to prove on the balance of probabilities that the COSTS (time, money and effort) of some precaution(s) to mitigate the risks were "disproportionate" to the reduced RISK that would arise from putting in the precaution(s).
The difference between what is "disportionate" and "grossly disproportionate" is very rarely put to the test as usually doing what is "reasonably practicable" is done without complex calculations but rather by the application of industry practice with an eye to guidance from the regulators and others.
However, if you read HSE's thoughts in "R2P2", then you might be thinking about what "Disproportionality Factor" - DF - might be appropriate in certain circumstances.
In R2P2, HSE suggest that a DF of more than 10 is "unlikely" to be reasonably practicable.
So, if the official UK Value for Preventing a (statistical) Fatality is currently just over £2m, that might mean that to prevent a fatal accident you might think of spending £2m x a DF of 10 = £20m.
....and you might get much more socio-economic benefit including to human health from spending that £20m elsewhere!
But if you base your thinking on the Marshall judgment you would be very unlikely to contemplate a DF as high as 10 and your comparison of decisions as to how to spend a few million would be made on an entirely different approach.
Perhaps a maximum DF of 2 or 3 to prevent a fatal accident at work = £4-6m. Lots left in your £20m kitty to spend on other things.