I have followed the suggestions posted elsewhere: Switch off router. Switch off Sonos. Switch off anything else on network - including Drobo. Then re-booted router, added Sonos and then switched on Drobo. There was no difference - Sonos controller still cannot access the files. Other material on the server is instantly available so no issues with that.
I have checked system preferences to ensure the Firewall is allowing access and that seems good, o running out of thoughts as to what is causing the problem. Grateful for any thoughts.
Best answer by Stanley_4