I finally got round to replacing my knock sensors, I tried the approach here:
http://www.subaru-svx.net/forum/showthread.php?t=17952
I did work, but it was a lot of hassle... took a hour for my big fat hands to connect the drivers side sensor...
I'll post some photos if they came out ok (maybe it'll help somebody else save a few minutes here an there)..
Car sounded much better with the new knock sensors, and the CEL didn't come one (although it may need a longer drive to get enough data to detect a problem). There is still a tap on one cylinder, hopefully that's just old oil in making the hydraulic lifter stick (I forget the proper name of the bit) and it will clean up itself after a little drive.
Next I'm going to try to get it to pass smog and get the registration done.
I've got a lot of ambitious plans for this car, and very little time to work on it. I'm starting to think I should pay for somebody to the work rather than have it sitting waiting for me to get some free time...
Cheers!
Ian.