From June Tabor's wiki entry....


. One of her earliest recordings was in 1972 on an anthology called Stagfolk Live.[5] She also featured on Rosie Hardman's Firebird (1972) and The First Folk Review Record (1974). At the time she was singing purely traditional unaccompanied material but in 1976 she collaborated with Maddy Prior on the Silly Sisters album and tour, with a full band that included Nic Jones. It provided the launching pad that same year (1976) for her first album in her own right, Airs and Graces. She later joined again with Prior, this time using the name Silly Sisters for their duo. Starting in 1977 Martin Simpson joined her in the recording studio for three albums before he moved to America in 1987. (Simpson has returned from America to be a guest guitarist on albums in the 2000s (decade).) After his departure, she started working closely with pianist Huw Warren.


So she certainly worked WITH Martin Simpson and Rosie Hardman, but to say she accompanied them is probably not really correct.

Certainly the Plains of Waterloo does not seem to have been a 17c ballad given that the battle occurred in the 19c.