A Big thank you for your IT engineer who sorted out my wireless problem and both my computers.
What a joy now to use my computers without waiting half a day to start up.
Firstly, open System Preferences from either the Apple menu at the top left of the screen or from the Dock. Then click on the Network icon.
Once the network options are open, click on the WiFi connection on the left hand side and then click on Advanced.
You'll now see a list of all the WiFi networks your Mac has connected to, simply click on the one you want to remove and then click on the minus icon just under the list.
Now click on OK to close this window and click on Apply on the next window. You can now close System Preferences and your Mac shouldn't try to connect to that WiFi network again.