Dear Dr Bergstrom, The source is William Knowler, ed., The Earl of Strafforde's letters and dispatches, with an essay towards his life by Sir George Radcliffe. From the originals in the possession of his great grandson. (2 Volumes. London, 1739). The Strafford papers have been available on microfilm for several years and are much fuller than the extracts published by Knowler. I hope that this helps. With best wishes, Christopher Thompson