We are looking seriously at joining Netflix. We have fibre broadband with download speeds of 100Mbps, with a modem and a range extender, which is within about 800mm of the Freeview plus box. Am I correct in assuming that the Freeview box will supply the needed signal to the TV, which is 10 years old (although "HD ready"). Am I better to use an Ethernet cable from the extender to the Freeview box or rely on the Wi-Fi signal?
That is subject to how reliable a WIFI signal from the modem to the FreeviewPlus box is, if you can get good signal then use the wifi mode of the box to connect to Freeview OnDemand apps.
You should have no problem, however it’s best to test either options to see if you can get a suitable stream running.
Connect the UHF aerial to the box then the HDMI to the TV :)