Having paid for an IT specialist to isolate the problem I can confirm that Avast will not permit Slack to run on my PC. There is a conflict between the two causing my internet connection to drop out, requiring a restart only to repeat the process the moment Slack is opened again.
Slack was uninstalled and the situation improved a little but I still experienced dropped internet connection.
I have now uninstalled Avast on my PC and Slack now runs perfectly. Also there have been no dropped internet connections since then.
Avast is causing the problem and it has cost me money to troubleshoot the issue to say nothing of the remaining months I have left on my subscription for an app I cannot use.