Wireless Power output?

[#1]

With the Plusnet Fibre router is it possible to increase the wireless power output? I want a little more poke from the router to reach the back yard.

[#2]

not on any of the supplied routers but you could pick up a third party router capable of this or one that can run ddwrt

[#3]

Depends which router you have - there's some wireless tweaks on the TG582: http://npr.me.uk/telnet.html#tweaks

[#4]

You don't say what you are using as client devices. Increasing the Tx power of the Access Points is only 1/2 the story.
Typically battery powered client devices only run at lower Tx power levels, this becomes the limiting factor.
You may be better looking at the antenna you are using, and trying a higher gain omni or directional. Or better still a second AP.