I have read Dominic Cumming's "explosive intervention" after No. 10 had clearly (if anonymously) briefed the press suggesting he was the source of the leaked Dyson/Johnson texts.
You can read it too at https://dominiccummings.com/2021/04/23/statement-regarding-no10-claims-today/
I believe him, for what it is worth.
The BBC's coverage of the story is hilarious because it is the BBC who broke the original story and to whom - one presumes - the texts were leaked. Plainly someone somewhere there knows the source.