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