How to extend broadband network to your garden office

A helpful 'How to' at PC Advisor on this very subject in answer to the following question:

I've recently built an extension on my kitchen and a home office at the bottom of the garden. My Wi-Fi signal doesn't reach the garden office, even with a wireless extender. I tried using HomePlug, but it seems that the office is on a different mains circuit to my router. Can you help me get an internet connection to my office with the minimum expense? I don't want to have to dig up the garden to run cables.

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