pcolbeck is 100% correct. I feel somewhat a berk for not being able to get hold of the idea that the EoP side could function just fine with an un/mis-configured admin side. My vain pride clings to the fact that the only documentation supplied in the box or by support makes no mention of the 192.168.x.x range. Somehow I had it lodged in my head that if the EoP side functions without the endpoints having an IP address the management interface must do too. Both logically and empirically bollocks.
It's management interface now resides in my 10. range.
Thank you all and pcolbeck in particular.