Wireless N blocks local File & Print sharing

[raetsel]

We recently switched to O2 Pro with Wireless Box III ( still in my 30 happiness guarantee in fact ).

We have two Macs, one Dell (Windows7) PC and 2 iPhones & 1 iPad .
Connections for my computers & devices to the Internet are fine with good download speed for my line quality.

However they cannot communicate with each other locally for file or print sharing over Wireless 802.11n. They cannot even ping each other ( though from the command line for the router I can ping them separately).

If I switch it to 802.11g then everything is fine.

So to recap: No problem any device connecting to the Internet over wireless n but they cannot connect to each other. ( Wireless g or Ethernet cable and they can talk to each other & the net).

I'm thinking some sort of weird router firewall issue (no PC or Mac runs any software firewall) but can't track it down.

The main reason I went for O2 Pro and the Wireless III box was to be able to use "n".

I've logged a call with O2 but not heard back yet. Their 24 hours are just up as I type this.

Any ideas please to get n working?

Thanks.