Could you give an example fo the sort of thing I should be looking for then?
Ouch, big topic. See this thread
http://www.pinkfishmedia.net/forum/showthread.php?t=148234 where I canvassed opinions and see what I did. Actually I still have a few options to try out but here are some thoughts.
In a nutshell, I believe that especially switch mode supplies
tend to dump noise onto the mains which can cause problems for hi-fi kit. Definitely, any digital transports should be placed behind their own filters: there is no downside in doing so. In my personal view, each analogue hi-fi component (including DACs) that uses a switch mode supply should be placed behind its own filter. Some people argue against it, especially for power amps, but I would try it for your speakers (one filter each) since they use class D power.
Check your consumer unit and see what other sockets are on the same ring as your hi-fi. As possible, relocate appliances (lamps, chargers, computers etc) to another ring. For what remains, place the appliances behind filters as possible.
If and only if ALL switch mode power supply items on the ring (hi-fi and non-hi-fi) have been placed behind filters, then I would experiment going filterless for the analogue hi-fi components that use linear power supplies. Some say this is on balance beneficial. (But if ANY unfiltered switch mode power supplies remained on the ring then I'd feel forced to put analogue hi-fi components that use linear supplies on a filter, though IMO they could share one without too much heartache.)
I have found the cheap Tacima filtered extensions to be effective.
Finally, buying decent branded extensions without indicator lamps or switches seems to me an inexpensive and sensible move e.g.
http://www.nexusinds.com/masterplug/uk/indoor-power-leads#1gExtLeads.
