The first bit of circuitry I've made in many many years (I am mostly virtual these days), and it worked! Here it is. If you're a circuits person you will sneer at its simplicity:
What does it do? It takes the headphone signal from a laptop and reduces the audio to the right level for plugging into our hi-fi. It has a switch (a DPDT, the shaded box) to nicely silence the sound without the hi-fi going buzzzzz
The tricky thing is that "headphone level" and "line level" aren't very rigidly defined. The circuit reduces the signal by a factor of 52, but at first I read on the internet that reducing it by about 5 would be enough. Did some tests with a voltmeter and 5 was nowhere near enough.
(By the way: we were thinking about getting a wifi-audio thingy (eg airtunes) to send sound around the house. Then we realised that wifi-audio has crappy latency, besides being expensive. This little box cost about 7 quid for all the parts...)