The link to the iPlayer radio archive is here. The show lasts for three and a half hours! But if you want to skip to the bit where Paul talks about SR-71s, A-12s and so forth, the skip to roughly 2 hours and 35 minutes in.
Paul's work for Osprey includes the fantastic Lockheed A-12: The CIA’s Blackbird and other variants, Lockheed SR 71: Operations in the Far East and Lockheed SR 71: Operations in the Far East.
Without giving too much away, I think I can get away with saying that Paul has a few very exciting Air Vanguard titles on the way - so keep an eye out for them!