Clients fails to netboot

I am seeing some strange behaviour with our new macmini, basically we have been using an old Xserve to netboot clients but have just recently setup a new macmini server an enabled the netboot on this. It's already running a few other services such as profile manager and software update and performing great.

I've created some netboot images, transferred some from our old Xserve and also setup some on deploystudio. The problem is, when I boot up a client and go to Sys Prefs > Change startup disk, I can see all the netboot images and if I select a netboot image, the machine reboots, I see the blinking globe and then after a few minutes it boots into the local hard drive.

I've also held down the N key to boot in like I would previously do but similar to above, it boots straight into the local HD. If I old down option, I can see the local HD and the recovery partition but no netboot images (strange? I see them within system preferences!)

Looking at the logs I dont seem to get the BSDP Discover but only this;-

06/03/2013 15:14:33.518 bootpd[253]: BSDP INFORM [en0] 1,c4:2c:3:1c:46:79 arch=i386 sysid=iMac11,2
06/03/2013 15:14:33.518 bootpd[253]: NetBoot: [1,c4:2c:3:1c:46:79] BSDP ACK[LIST] sent IPADDRESS pktsize 300
06/03/2013 15:14:36.147 bootpd[253]: BSDP INFORM [en0] 1,c4:2c:3:1c:46:79 arch=i386 sysid=iMac11,2
06/03/2013 15:14:36.147 bootpd[253]: NetBoot: [1,c4:2c:3:1c:46:79] BSDP ACK[LIST] sent IPADDRESS pktsize 300
06/03/2013 15:14:40.950 bootpd[253]: BSDP INFORM [en0] 1,c4:2c:3:1c:46:79 arch=i386 sysid=iMac11,2
06/03/2013 15:14:40.950 bootpd[253]: NetBoot: [1,c4:2c:3:1c:46:79] BSDP ACK[LIST] sent IPADDRESS pktsize 300
06/03/2013 15:14:49.078 bootpd[253]: BSDP INFORM [en0] 1,c4:2c:3:1c:46:79 arch=i386 sysid=iMac11,2
06/03/2013 15:14:49.079 bootpd[253]: NetBoot: [1,c4:2c:3:1c:46:79] BSDP ACK[LIST] sent IPADDRESS pktsize 300
06/03/2013 15:15:06.047 bootpd[253]: BSDP INFORM [en0] 1,c4:2c:3:1c:46:79 arch=i386 sysid=iMac11,2
06/03/2013 15:15:06.047 bootpd[253]: NetBoot: [1,c4:2c:3:1c:46:79] BSDP ACK[LIST] sent IPADDRESS pktsize 300
06/03/2013 15:15:23.008 bootpd[253]: BSDP INFORM [en0] 1,c4:2c:3:1c:46:79 arch=i386 sysid=iMac11,2
06/03/2013 15:15:23.008 bootpd[253]: NetBoot: [1,c4:2c:3:1c:46:79] BSDP ACK[LIST] sent IPADDRESS pktsize 300
06/03/2013 15:15:27.332 bootpd[253]: server name macmini.lifesci.dundee.ac.uk
06/03/2013 15:15:27.332 bootpd[253]: interface en0: ip IPADDRESS mask 255.255.255.0

I had our networks guy helped me with using tcpdump to see how the server responds and we only got, he also assures me that the actual server is not sending packets out and suggests its the servers fault? ;-

15:37:13.823326 IP vl316.name1-dist1.net.address.ac.uk.bootps > server.address.ac.uk.bootps: 
BOOTP/DHCP, Request from 10:dd:b1:a7:a7:2f (oui Unknown), length 286

(I know the mac address is different as we tried different machines)

I was also advised that we use a helper tool within our VLAN to distribute IP's which was setup on our Xserve and also setup correctly with the new server along with the same ACL's that were copied across.

Ive ran out of ideas on what to test, would appreciate if someone has anymore testing tips or ideas?

Thanks