I have tried re-installing the current FW, no difference.
I have restored factory default, the page re-appears but once I restore my settings the problem returns.
I did wonder if it might be a case that as the router is the DHCP for 4 VLAN that this might be causing the trouble. If not what other settings might cause this (given that it works when I factory default, it seems safe to think it's a settings issue).
Many thanks.
Originally listed in wrong place.