Success !

Ratbert suggestion worked.

I plugged an Apple Time Capsule into the Belkin router and wired both the iMac and the MC200 into the time capsule with ethernet. Since the Time Capsule supports Multicast it works.

Meridian Sooloos Configure program on Win-7 laptop also now sees the MC200 if I plug the laptop into the Time Capsule with ethernet.

Note that neither the Windows laptop nor iMac can see the MC200 via wifi because they connect to the wifi on the router whereas I have the ethernet cables going into the Time Capsule and it's only the Time Capsule that supports Multicast. If I swapped the Time Capsule to be the wifi base station instead of the router I expect it would work on wifi.

Thanks everyone especially Ratbert.
Will now turn the firewalls back on, fingers crossed.

Regards
Shaun

G65, DSP3200, DSP3300, MC200, HD621.