I use 500Mb/s powerline adaptors between HH5 and my old HH3. (You can get pairs for about £20 for fast ethernet or about £35 for gigabit ethernet. NB fast ethernet is fine. don't waste your money!)
This is a very effective way of spreading the WiFI load. BT's home hubs don't like more than about 4-5 devices connected simultaneously,
My home's WiFi connectivity and throughput has been totally transformed.
I am also interested in doing what Antony is intending. I have a HH5 and Infinity for my main network and a couple of TP link mains relays to boost the signal but I have one room, on the other side of a metre thick cob wall, that has no signal. I have run a network cable in there and can connect via wired but would like to provide a wireless connection also. BT left me the HH3 when they switched me to Infinity so I have tried to configure it as suggested in earlier posts. However, it seems to insist that it have a broadband connection and intervenes in any Internet access, demanding that this be fixed. Oddly enough Windows seems happy with the signal and tells me that I have Internet access but I can't get to the Internet without the HH3 interrupting. I have the LAN cable from the HH5 plugged into one of the yellow ports. How can I tell the HH3 not to bother with the broadband connection? I can see nothing in the settings to circumvent this.
When you log on as admin, your HH3 should show Internet as 'Disconnected'. Just leave it that way.
Just use the LAN ports.
Thanks for the response. Sorry for delay in replying - was working away last week. The HH3 does show Internet as Disconnected.
Got round the problem with laptop by connecting via a cable through the HH3 with the WiFi off on the laptop. That gave me a wired connection to the Internet via the HH3 to the HH5. Then turned on WiFi and connected it to the HH3 and disconnected the cable. The WiFi connection then continued connecting via the HH3 which connects by cable to the HH5 and thence the Internet.
Phone had original problem however and kept coming up with the HH3 screen asking to fix the lack of broadband. Moved out of range of the HH3 and connected to the HH5 Wifi then came back into range of the HH3 and that seemed to fix it.
It seems that there has to be an established connection to the HH5 otherwise the HH3 intervenes and forces you to try to resolve the lack of broadband.
Addendum to my previous comment.
Although I had set up the HH3 as instructed for the HH3b in Keith Beddoe's excellent guide, I later reset it to factory state and forgot to turn off the DHCP when reconfiguring. This caused it to intervene and demand that I fix the broadband even when I was connected via wireless to the HH5.
I also wanted to use a home hub 3 to extend my wireless to a dead spot.
Heith Beddoe's excellent article here - http://forumhelp.dyndns.info/wireless/hh3link.html gave me all the info I needed. It took ten or fifteen minutes to set up my home hub 3 and it works perfectly. I'm going to save the instructions in case the web page goes some day.