May I ask why the urgency?
The site is well recorded in Hall's 'Metal Mines of southern Wales' and Foster-Smith's 'Metal mines of the south Wales area', including NGRs and summaries of the workings (but no plans). Peter Claughton's mining history website contains useful information: google 'Peter Claughton' and 'Trelech'.
It can readily be located by comparing the historic OS 1:10,560 maps on the National Library of Scotland website with current OS maps (1:25,000 available for free on Bing maps), from which you can derive an accurate NGR.