Powerline adapters are great for people like me who live in rented accommodation. Why in the heck would I go to the expense of installing Cat-5 throughout someone else's house?
For Fareastdriver, in option 3 I was suggesting that you use a pair of powerline adapters to connect the BT and Talk Talk routers together. One of which would be connected to ADSL, the other configured to act as a Wifi-Ethernet bridge. It's not as elegant as a wifi extender, as you end up with two wifi networks, and in some situations might have to be careful to remember to switch from one to the other. But cost wise might be cheaper than getting a wifi extender. It also presupposes that one of the routers you have, can be configured to act as a bridge.
Cost wise the first option would be the cheapest if you can find a better location.