Just an update.......
I brought the car to a dealer yesterday and the following fault codes showed:
P0137 - O2 Sensor Circuit - Low Voltage Bank 1 Sensor 2
P0410 - Sec Air system function
P2444 - Sec Air Pump malfunction
P1410 - Sec Air cab valve siezed/open
When they took a look, the wiring loom also has water problems. I was asked if I had steamed cleaned the engine, and I hadn't, but that's not to say the previous owner hadn't done it.
2 hours were spent taking the valves out, cleaning them, taking the wiring loom apart, drying that out, and putting the lot back together.
The fault codes were cleared and they went for a test drive. All went well, no CEL light.
However, on the way home, about 10 miles from the garage, the fookin light came back on again.
So, they are going to ring Subaru today to see if there is anything else that they can do before they recommend replacing the lot.
I was quoted 1280 to replace the valves but the car may need a wiring loom as well.
I think I can get the parts second hand even though I have read some unfavourable stuff about "24/7 spares" after doing a google on the phone number they've given me.