Any expenditure outside the annual investment allowance will attract writing-down allowances of 8% on integral features, 18% on general equipment and 100% for specific energy efficient plant.

Tight cost control has been crucial too, particularly the consolidating of its factories into fewer, more efficient plants.

The most efficient plants, which are almost invariably the least costly to run per kilowatt-hour produced, are brought online first.

That number began to decline during the 1960s as companies closed in favour of newer, more efficient plants elsewhere.