Ok i am using firestarter for the firewall.
I have added the following to the user defined section of the firewall:
$IPT -A INPUT -s 224.0.0.0/8 -d 0/0 -j ACCEPT
$IPT -A INPUT -s 0/0 -d 224.0.0.0/8 -j ACCEPT
$IPT -A OUTPUT -s 224.0.0.0/8 -d 0/0 -j ACCEPT
$IPT -A OUTPUT -s 0/0 -d 224.0.0.0/8 -j ACCEPT
These are pre applied when the firewall starts up, so they are on top of the input and output chains.
Correct me if I am wrong but that should all all multicast both in and out of both networks?
I have found that this problem reproduces itself within an hour and fifteen minutes so i still have 45 minutes to wait to see if this fixes it. Will let you know.