This thread suggests it works without changing any settings. That IPTV setting doesn't feel right for this - it may work with just IGMP Snooping turned on.
I've cancelled the order for the router now anyway, last thing i need is the wrath of the missus because the TV doesn't work on both boxes.
I'm cancelling multiroom in 3 months and by then the router i ordered will be off its first firmware and a more mature product, knowing it already works with a single box will be just fine by then but thanks for the help anyway.
And you never know i might have worked out the mystery of the failing 5GHz on the HH5 Type B by then, i know i am odd but i actually quite like the HH5.