Remotely monitoring ISC DHCP Server with Nagios

I have a WAN and I'm trying to use Nagios to check that a DHCP server in a subnet not directly visible to the Nagios box is up and handing out IP addresses, and I've been unable to make this work.

I have 3 places I can monitor DHCP from, but none seem to be working. The ideal would be to have Nagios monitor DHCP directly, failing which I would like to ssh to the DHCP server and check it from that box, failing which check its working from a core router with an interface on the DHCP server.

The DHCP server is used exclusively to dynamically assign static IP addresses to servers.

The DHCP server is on 192.168.254.1, and is running ISC DHCPD.

My dhcpd.conf file (currently) reads as follows:

# Long Lease times - 1 month, 1 year !
default-lease-time 2678400;
max-lease-time     31622400;
authorative;

subnet 192.168.224.0 netmask 255.255.224.0
{
    range 192.168.225.1 192.168.225.2;

    option domain-name-servers 192.168.200.254, 192.168.100.254, 8.8.8.8;
    option routers 192.168.254.254;
    range   192.168.253.0 192.168.253.0;

    host web.mydomain.co.nz
    {
            hardware ethernet 52:54:00:F3:80:32;
            fixed-address 192.168.242.2;
    }

    host nagiostest.mydomain.co.nz
    {
            hardware ethernet 52:54:00:00:00:01;
            fixed-address 192.168.254.250;
    }
}

web.mydomain.co.nz is working fine – ie DHCP is correctly assigning it an IP address.

When I run (from the DHCP server) any of the following commands I get a "no answer" reply –

dhcping -v -h 52:54:00:00:00:01 dhcping -v -s 192.168.254.1  -h 52:54:00:00:00:01 
dhcping -v -c 192.168.254.250  -h 52:54:00:00:00:01 
dhcping -v -c 192.168.254.250 -s 192.168.254.1 -h 52:54:00:00:00:01

When, from the server, I do a tcpdump -n -i eth0 port 68 I don't get any kind of response.

When I do something similar from a machine on another part of the network (located 2 hops away at 10.10.10.11) or when I try similar commands from the gateway I see the request coming in to the router, but no response – TCPDump running on the DHCP server shows –

   tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
08:53:28.103926 IP 192.168.254.254.bootpc > 192.168.254.1.bootps: BOOTP/DHCP, Request from 52:54:00:00:00:01, length 250

This happens even when I drop the firewall on the DHCP server altogether.

Using the check_dhcp plugin for nagios fairs no better.

How can I check to see the DHCP server is up and handing out leases ?