Found the problem. Tried curl as you suggest and the DNS lookup failed. However the nslookup command could resolve the address. I flushed the DNS cache / restarted DNS (?): sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder and all is now working as expected. Thanks for your guidance and encouragement.

Thanks for the reply that makes sense, particularly as further investigation shows that only one of eleven machines on the same subnet are having this issue. I have compared configuration between working and non-working machines and can not see any differences. I have also sent a network capture to a network specalist. Do you have any other thoughts on what might cause this behaviour?