I've just checked for fault codes and it's returned fault code 22 which is 'knock sensor' car is a '99 classic, does the code mean the 'knock sensor' as detected knock or does it mean the sensor is faulty?

 

Normally faulty knock sensor. Although I had it on mine a little while after changing the fuel pump. Ordered the new sensor from subaru, but reset the ECU before, and the fault hasn't come back after over 3 years. So give that a try.
I have a PSi3, so its easy for me to detect codes and reset ECU.

 

I've reset the ECU but I've yet to recheck it, I'll do it tonight. this may sound stupid but what's a PSi3?

 

Its an ECU monitor that was made for Subaru's. Tells you all kinds of real time information based on ECU readings. handy for reading codes with actual descriptions, and resetting the ECU with the touch of a button.
The firm went bust a while back, so they are hard to get hold of now.

 

OK, I guess I'll have make do with plugging and un-plugging wires

 

Still a good way to do it. Free diagnostics are always good.

 

Refresh the connection on the knock sensor (clean the pins in the plug with contact spary)
Then reset the ecu and see how it goes BEFORE you part with your hard earned buddy.

 

I'm not complaining the system works very well, and free dianostics is deffo worth a bit of fumbling under the dash.

Good idea it could be simply a loose or dirty connection.