I spent some time looking for that in the late 70s, without success. I had a rough idea of which hillock it was in but I don't remember coming across a steel sheet.
The most useful reference I came across is Dave Elliott's "Caves Of Northern Derbyshire" Part 1; The Eldon Hill Area (Jan 1975) pages 1 & 2. This includes a useful elevation and also a location map. It gives the location as "about 200 yds west of Bull Pit (NGR107814) and around 15 yds south of the B6061 Sparrowpit/Castleton road. This also tells us that it was formerly known as "SP Mine by potholers. (So searching for this, as well as Cockshead Mine, in caving publications may be useful?)
There are further references given in this article, the most useful of which will probably be:
Chandler B, 1953, British Caver V24 pp 73-75 (inc surveys)
Ford T D, 1966, BSA Cave Science 39 (p382).
Dave Elliott's reference list (in his CoND article above) includes "Speleogation Records" - as do many other entries in the 5 part series. I've often wondered what these Speleogation Records consist of; they may include a lot of interesting stuff. I bump into Dave now and again so I'll ask him next time I see him. (Or - Dave - if you read this, can you enlighten us?)
You could try asking Trevor Ford directly? (Best method likely to be by letter, with an SAE enclosed.)