Will the installation engineer be happy to run either the master-modem or modem-HH cable from the master socket upto an attic room? I have my existing modem/router/wifi there as most of my kit is there and the guidance is that wifi is best broadcast down (we also have a homeplug circuit to reach where the wifi still doesn't).
If he/she won't, I presume I could move the HH afterwards and use a Cat6 or 7 cable?
Provided that the wiring is in good condition, the engineer could make the socket up in the attic room the Master and the current Master into and extension. You can then maintain the setup as it is.
I have my modem next to the master socket, and I've run my own network cable to the HH3 which is in my kitchen to give a better wireless signal in the house. The engineer said he'd only ever used the 'red' cable that comes with the infintiy starter kit so didn't think it would work, and was pleased/surprised when it did - said he'd learnt something!
So my advice is run your own network cable to where you want the hub and keep the modem next to the master socket.
And if I go for the modem at master with separated HH, it looks like Cat6 cable is ok for the distance.
I'll see what the engineer is willing to do.