I’ve now found the cause of the problem. I have a second Linkstation, Live version, and when that is on I need to have Bonjour running on the pc for the soundbridges to see the firefly server on the Linkstation Pro. When the LS Live is off, Bonjour is not needed. Not a router problem.
I have the LS Live running only 1 day a week to do a LS->LS backup so I can live with this. Strange behaviour though. Pcast is disabled on LS Live.
EDIT:
Please disregard the above, bad analysis on my part. I hadn’t realised that although the pc laptop is in sleep mode it is still responding.