Yeah, wish i could be out during the week too.
If i'm reading Mike Gills book correctly it was started around 1802, by 1811 it was 3100ft long and was part of a plan to develop the eastern end of the Fryerfold Vein.
Driving the Barras End level up to the Surrender Boundary was a piece of 'dead work' that was a stipulation of a lease granted in 1811.
It was driven from 1811 to 1814, then again from 1820 to 1823. Finally in 1828 it was driven the remaining 360ft to the Surrender Boundary. The Surrender company then drove it some distance into their ground along the Barras End Vein.
A few web searches mention 1km of caverns, but I have no idea if they are still accessable.
Also the following if anyone has a copy ?
Thorne, P. "Orpheus Caving Club Swaledale Meet, Easter 1985 (Moulds Old Level and Barras End Low Level)" Newsletter of the Orpheus Caving Club, 21:4 (1985), pp.13-17.
There are a dozen references to it in these archive records too
http://www.nationalarchives.gov.uk/A2A/records.aspx?cat=191-zlb_2&cid=-1&Gsm=2008-06-18#-1