Quick update - I think I have sorted it.
I have connected the mynet box via the yellow internet terminal to a spare model/router ethernet port.
Did a system reset to make sure it was all clean.
Set the mynet box to access point mode (menu/advance/admin/device mode).
Switched the wireless (both) off.
Need to keep the ethernet connected for this to work.
Not sure if this will stop the reboot, only time will tell, but at least I have the use of the hard drive back on my network and still be able to connect to the internet through the original model/router.
Couple of quick checks -
Play video over the network to PC and tablet OK
Copies 66Mb video file from mynet to pc over wifi at 10Mb/s
Copies 66Mb video file from pc to mynet over wifi at 7Mb/s
Acceptable and slightly better than i had just using the mynet !
I’ll report back if anything changes.