It speaks, but my PC has its fingers in its ears

I almost got UDP working this evening. I wrote a simple Python test script to send a "get status" command to HydraRaptor. Here it is :-

   from socket import *

 # Set the socket parameters
 hydra = "10.0.0.42"
 inPort = 21000
 inAddr = (hydra,inPort)
 outPort = 21001
 outAddr = ("localhost",outPort)
 buf = 1024

 # Create sockets
 sendSock = socket(AF_INET,SOCK_DGRAM)
 recvSock = socket(AF_INET,SOCK_DGRAM)
 recvSock.bind(outAddr)
 recvSock.settimeout(2.0)

 # Send a get status command
 cmd = "\x12\x34\x56\x78\x00"
 if(sendSock.sendto(cmd,inAddr)):
     print "Sending message ....."

 # Receive the reply
 try:
     data,addr = recvSock.recvfrom(buf)
     if data:
         print " Received message '", data, "'"
 except:
     print "Read failed"

 # Close sockets
 recvSock.close()
 sendSock.close()

HydraRaptor replies with its x,y,z coordinates and the number of steps remaining. The reply packet arrives at my PC which then sends an ICMP destination unreachable (port unreachable) message back. I have no idea why. netstat -ad shows :

  UDP    shuttle:21001          *:*                                    2412
[Pythonwin.exe]

Which looks to me to show that the port is being listened to. I have tried turning my firewall off but that made no difference. I am sure the problem is at the PC end because Ethereal shows me that the packets are well formed. I have had enough for this evening. Any Python socket experts out there? I have never done sockets in Python before.