Dave, you need to be aware of an issue with the 600 units, and possible the others, as I think you are using the TP Link router. This can be easily overcome as described on this link.
Its more of a problem if you try and add additional adapters, as I suspect other BT ones may have the same issue with their default startup address.
I cannot see the 300 or the 500 being sold on the BT Shop any more.
My daughter has the mini home hotspot 600 multi kit, then added two more hotspots from another kit, keeping the other master unit as a spare. So she has a hotspot in each room now, and that works well. In addition she gets two Ethernet ports in each room.