Hi i have 2 PLCS and a HMI in a system using TIA portal v19, i need to synchronize all 3 devices to the same external NTP server, do you just point each device at the device with the link to the external NTP server or is there a more complex procedure to follow?  

Thanks in anticipation for any advice

You set the NTP server in each PLC and HMI. Here's a document that refers to 1500 series.

Time synchronization with an online NTP server for SIMATIC S7-1500 Controller

Apologies for the delay to respond, i still have questions and i will try to be more direct with what i am asking.

I have 2 PLCs and 2 HMIs that require synchronization with a NTP Server, 

PLC01 - this has a seperate network card on a differenet IP range that connects to the network where the NTP server resides so NTP settings in the project should be straight forward as i can use “Set NTP server in the project” and enter the I.P of the said server

PLC02 - this is on the same machine network only, can i use “Set NTP server in the project” and enter the I.P of PLC01

HMI01, HMI02 - this is on the same machine network only, can i also enter the I.P of PLC01  

in essence what i have is 1 PLC that updates from the NTP server and the others use that PLC as their NTP server

You can program an S7 PLC to be a SNTP server using this library:

Library for SNTP Server Functionality in SIMATIC S7 CPUs

Then you can point your other devices to the IP address of that PLC, as you described, and it should work to synchronise their times. I used the SNTP server from this library once in a project a few years ago and it worked fine, at least for S7-1500 PLCs (I didn't have HMIs in that project)