Both applications are running on the same laptop, how do I set-up the receiver so it can receive messages broadcast on the same IP and port 30119 ?

Do you want to bind each socket on same port?
So you have allow reusing same port:

if(!pUDPsocket->bind(QHostAddress::LocalHost, 8124, QAbstractSocket::ShareAddress | QAbstractSocket::ReuseAddressHint))
{
   qWarn() << "Failed to bind UDP socket:" << pUDPsocket->errorString();
}

The problem I'm having is the laptop is so locked down by the company IT that I'm not sure if it is causing problems, as I said, last week these same applications were communicating.

And therein is the problem...and it ticks me off to no end. If you cannot own and control your dev resources then you're just whizzin in the wind when you need to do real work. These dumb-acre kids who think they know and understand network security need to be taken to the wood shed and whipped with a willow switch when they start treating devs like office admin workers.

Both applications are running on the same laptop, how do I set-up the receiver so it can receive messages broadcast on the same IP and port 30119 ?

Do you want to bind each socket on same port?
So you have allow reusing same port:

if(!pUDPsocket->bind(QHostAddress::LocalHost, 8124, QAbstractSocket::ShareAddress | QAbstractSocket::ReuseAddressHint))
{
   qWarn() << "Failed to bind UDP socket:" << pUDPsocket->errorString();
}

Failed to bind UDP socket: The bound address is already in use

Both applications are running on the same laptop, how do I set-up the receiver so it can receive messages broadcast on the same IP and port 30119 ?

Whats preventing me choosing a port that is in use for something else?

From the link you posted (and what I wrote before): "On Unix-like operating systems, a process must execute with superuser privileges to be able to bind a network socket to an IP address using one of the well-known ports."

Whats preventing me choosing a port that is in use for something else? For example 13?

As already written by @jsulm , port numbers bellow 1000 are reserved for system. You need to have root rights to bind on them.
This is called security ;)

Whats preventing me choosing a port that is in use for something else? For example 13?

Failed to bind UDP socket: The address is protected.

pUDPsocket->bind(QHostAddress::LocalHost, 8124);

You should verify that bind did work:

if(!pUDPsocket->bind(QHostAddress::LocalHost, 8124))
{
   qWarn() << "Failed to bind UDP socket:" << pUDPsocket->errorString();
}

The connection source for this application:

pUDPsocket = new QUdpSocket(this);
pUDPsocket->bind(QHostAddress::LocalHost, 8124);
QObject::connect(pUDPsocket, &QUdpSocket::readyRead, this, &SckServer::udpRead);

My slot:

void SckServer::udpRead() {
    while( pUDPsocket->hasPendingDatagrams() ) {
        QNetworkDatagram datagram(pUDPsocket->receiveDatagram());
        ...
    }
}

It never gets into udpRead... The IP address resolves to 10.132.55.189 and the port is 8124.

not receiving it on the other application.

On which port+ip have you bind you udp socket?

The problem I'm having is the laptop is so locked down by the company IT that I'm not sure if it is causing problems, as I said, last week these same applications were communicating.

I will try capturing data on WireShark for the UDP port.

I've just added filter to WireShark:

udp.port eq 8124

8124 is the port I'm using to broadcast on, I've also modified my source to:

writeDatagram(bytaryDoc, QHostAddress::Broadcast, suint16Port);

Seeing nothing in WireShark.

Since the function returns a positive value indicating that It has succeeded, what else can I do?

That's not totally true, positive return value only indicates that message has been "accepted" by TCP/IP stack, that's all.

Where are receiver and sender located? on same machine?
Are you sure you are using right network support?
Do you have multiple possible routes between receiver and sender?

For WireShark you can start an listening for the UDP port you are using for your "heartbeat" signal. In filter simply enter "upd port xxx" where xxx is the port number (cf https://wiki.wireshark.org/CaptureFilters for more details).