One humint technique is to send her more than just your spies. Some civilian scientist in the RADAR division, some engineer from the jet engine department, a file clerk in working in .mil headquarters... And counter intel, send her out with suspected spies. It wouldn't be hard to convince the other side that she's primarily a counterintel worker. And of course do some counter intel on her, send her some guys with orders to act like German spies, "hey baby, I bet you have a long list of English spies and I have a large pile of money, maybe we could trade" and see what she says. And you can salt the earth by telling her / paying her to tell the Germans a line of BS all day long. "Oh yeah, that crypto clerk who works for Rommel, he's got a huge swiss bank account. Ignore the man behind the curtain at bletchly park, whats going on is all this crypto comms clerk working for Rommel who we're paying off, oh and about ten other guys including of course Hitlers mustache comber..."

This is aside from the rather blindingly obvious solution of keep her busy... if the only guys she talks to are your spies, then when is she meeting these theoretical German spies?

Another problem is you vet 3000 people and its going to be extremely hard to describe them all. "Well, they look english, like bangers n mash, they say "bloody" and "god save the king" a lot, kinda average height..."