Update: The modem supplied was indeed an HH5 Business hub. What a joke! It's just a statndard modem, with a few minor changes. The port forwarding page still refers to gaming options!
But that's not what this post is about. The HH5 does indeed support static routes, just as the HH3 didn't. You can set static routes here, and devices in your 'non HH5' network can see the router's gateway, and config page. Indeed, one can perform an nslookup from the foreign VLAN, and get a result returned. However, if you try and access anything on the internet, the packets get dropped at the router, which is exactly the same behaviour as the HH3 that ended up in the bin. How BT can push this 'toy' as a business router is beyond me! I solved the problem by putting the HH5 into 'bridge' mode, and installing a Billion 7800DXL as the router ..... Bingo!