:Mark Nicholls in the Oxford Dictionary of National Biography (2004) agrees with you. In his article on Catesby he says  
 :Nicholls cites this to "TNA: PRO SP 14/216/136", although he doesn't explain exactly what that source is. I can't trace it online, except that Nicholls uses it again in his 'Strategy and Motivation in the Gunpowder Plot', in The Historical Journal 50 (2007), 787–807.