Alain, thanks for responding.
Your response set me to thinking what the hell else could be causing this.
Then the penny dropped - I have 2 identical webservers with different names running in the same AWS EC2 (Windows 10) instance - managed by an external AWS load balancer.
Looking at the load balancer there is an attribute "Connection idle timeout" set to 60 seconds - if I change that to 120 seconds the reload interval adjusts accordingly.
All is good...