Working with JSON responses in Velocity

We've got a webservice that's returning a JSON result.. however, I'm struggling to get to data within the JSON response using Velocity. I can see it's a JSON object in the sandbox, and I've tried using ParseJSON to convert it to an object.. but I then can't do anything with that object.

Here's some sample code (I've put the JSON into a string for this example so it doesn't depend on the webservice);

#set($responseText = '{ "responseHeader":{ "zkConnected":true, "status":0, "QTime":82, "params":{ "q":"*:*", "indent":"true", "rows":"0", "wt":"json"}}, "response":{"numFound":653874,"start":0,"maxScore":3.5213852,"docs":[] }}')
responseText: $responseText<br>
responseText.response: $responseText.response<br>

#set($responseObj = $core_v2_utility.ParseJson($responseText))
responseObj: $responseObj
 

When this is run in the sandbox, it correctly shows the string is JSON (just like it does when it's got the payload from the actual webservice);

How do I get down into the JSON and pull out something like 'numFound' ?