Hi Guys,
Just an update to say thanks for all your help, the problem has now been resolved.
After removing the Belkin router from my setup and connecting the AirPort to the cable modem directly my emails now work fine.
So I can now definively say that the Belkin F5D7230-4 6000 with the latest firmwire does filter / block RPC over HTTP traffic.
My thanks to everyone who posted on this thread,
James