I've read up on the spacer before too. Its worth trying to find one. There was a chap on here that was going to run a few off but it never happened. M18 thread IIRC? I used to get this issue a long time ago with a standard downpipe and HJS 200 cpi sport cat. I then fitted a Hoffmann downpipe with 200 cpi sport cat and it didn't throw up the light as often. Now I have 2.5" downpipe again with a HJS 200 cpi sport cat and I haven't seen the light come on for over 2 years??
I bought myself a Gendan code reader in the early days so that I could just clear the fault instead of removing the battery lead every 600/800 miles.