Where was the Skripal leak?

The New York Times reported in May of this year that the "spook version of a lecture tour" that ex-Russian spy Sergei Skripal engaged in was kept secret and was known only to a select few intelligent agents. He gave talks to friendly spy services, but the Russians weren't supposed to know about them. The NYT mentioned 2012 as a year in which Skripal visited Prague on one such tour. In a report earlier this month, Janek Kroupa of Czech Radio added Oct. 2014 to the list and revealed that Skripal's later assailants were following Skripal while he was here, although they arrived in Prague before he did. Yesterday, Martin Dorazín of Czech Radio reported that the Russians are now looking for the leak about the travel itineraries of the two Russian spies. This ups the intrigue but doesn't fully resolve the Prague mystery. Where was the leak in the U.K. or Prague that allowed the Russian spies to know when Skripal would be in Prague so that they could "pre-follow" him here?