A bit late in the day but I only found the following out yesterday browsing the CRO. Offers an ideal opportunity to be side tracked once again!
Mining Record Office (Health and Safety Executive)
The most important Parliamentary legislation affecting the records of Cornish mines was the 1872 Metalliferous Mines Regulations Act...
...which required the owners of mines, on the abandonment of workings, to deposit plans, drawings and sections with the Secretary of State.
In accordance with the 1954 and 1969 Mines and Quarries Acts, and Mines Regulations 1956, all plans, drawings and sections relating to any abandoned mines or seams sent to a district mining inspector, or created by a surveyor, are required to be preserved by the Secretary of State, or by some other person under special arrangement. In November 1973 an agreement was drawn up by the Secretary of State, for Trade and Industry with the Cornwall County Council for the Transfer of those abandoned mines which related to Cornwall.
In 1989 the Health and Safety Executive, the current custodians of the plans, proposed their "decentralisation" to local authorities, and after consultation with the Public Record Office the Lord Chancellors' approval was obtained for their deposit in local record offices.
Available at the CRO (catalogue MRO)
Some 3500 plans for over 500 individual mines. These plans include some surface plans, but they are mainly underground plans, longitudinal sections and transverse sections. Negatives of most of them are available, which can usually be developed full-size if copies are needed.
Could keep one busy for months.
😎
The past is a foreign country: they do things differently there.