I have recently changed a customers router from a Netgear to a 2820.

Since the change the customers Squeezebox devices can no longer communicate with the Squeezebox servers on the internet.

A similar problem - the customer also has a configurable Logitech remote control that connects out to Logitech to get the device codes - this too doesn't connect.

All was fine with the Netgear

Any ideas appreciated.

depending on the ISP (talktalk especially), try changing the MTU, with 3.3.3 firmware that means accessing the telnet interface of the router and using this command:
wan mtu 1390
with firmwares before that you would use "wan ppp_mss 1390".
Also, try turning on UPNP if it isn't on already, that's under the Applications menu on the router, just enable all 3 options.