Just in case someone else has a similar problem - I took the axle out yesterday and had a closer look. The new sensor seats in their perfectly and the replacement I got off ebay was just too long. The FCP part is fine.
The tone ring was on the axle when I took it apart the first time, but had been rubbing the first sensor I put in, which was too long and when I popped the axle out this time, the ring just fell right off. It was no longer attached, so I pulled it off to avoid damaging anything else, put the axle back in, and will have to grab a new axle in a couple weeks when I get some extra cash.