I had thought the attribution to Croft was secure, but having googled around a bit today I found that the weight of musicological opinion is that the piece is based on a previous work by Purcell, but was composed by Croft. There is a Musical Times
article from 1978 in which the following comment appears, referring to the Ground in C minor:
...This keyboard transcription of the alto solo 'With him he brings the partner of his throne' from Purcell''s 1686 Birthday Ode for King James II [...] has been attributed to William Croft, appearing as the final movement of his Suite no. 4 in C minor.
(Meffen, John. “A Question of Temperament: Purcell and Croft.” The Musical Times, vol. 119, no. 1624, 1978, pp. 504–506. JSTOR, www.jstor.org/stable/959915
Howard Ferguson and Christopher Hogwood include the ground in their 1974 edition Complete harpsichord works / William Croft and it appears on several CD recordings of Croft's work. However I believe there are two manuscript sources (in the Fitzwilliam Museum and the British Library) one of which attributes it to Purcell and the other to Croft. I have altered the title of this thread to reflect this discrepancy.