It's OK folk. I solved the problem myself.
What most of the on line guides and videos don't tell you is exactly how to set up your AP Router IP manually. They tend to skip over that, just leaving you with automatic IP acquisition. 192.168.10. ***
My N66U IP is now 192.168.1.5 , manually set as part of setting AP mode. I can now access the UI no problem and it works perfectly as a media server too.
The key thing that seemed to make a difference was to set the default DNS server the same as the default gateway, i.e. 192.168.1.1 (The main router ip) I don't know if this applies to routers other than ASUS, because ASUS routers cache DNS.