I don't think the following will help as I know nothing about EFTPOS systems in the UK but here in AUS the basic concept is that once the transaction is initiated from the till system to the card terminal the process is entirely atomic and takes place between the customer and the financial switch system. One of the reasons is to prevent fraud by the operator. So I question whether you are actually getting these interim messages while the transaction is in process. Again here, all the till system receives is a notification whether the transaction was processed or it failed for any reason at all.
Also, signature authorisation is no longer used here. And I wonder just how many till operators in the UK have been trained on how to validate a customers signature?  
Here, any attempt to communicate between the till and terminal until the final result is obtained by the terminal will "fail" the transaction.
b

  thatbruce

Here we can have communication with the EFT PinPad but it is only from the PinPad to the PoS and all it is would be things like

Present card, PIN Entry, Connecting, Authorized etc (so the cashier can also see the status messages that the customer sees)

I can see the communication with no problems as it runs fine in the background the problem I have is nothing is being added to my listbox
until the HTTP work is completed and then the listbox refreshes and everything is showing up in the list (I am convinced it is something to do with how the thread is refreshing the form and the listbox)