Becky, a couple of other thoughts for you if you are still struggling with that afternoon nap..both of these are tactics I had to used consistently for a month or so to get DS to sleep well in the afternoons.
I did W2S **EVERY** nap for about a month. I had to sit in his room to watch when he fell asleep and check the time that he closed his eyes, because I needed to tiptoe in and touch his face, just until he flinched, at 22 minutes. I can not for the life of me remember how I figured out that it had to be at 22 minutes (probably a mom's desperation) but if it was 20min or 24 min then it didn't work. Had to be 22! Like I said, it did take a while of doing this to set his sleep cycles and it was a TOTAL pain to have to go back in after I just got him to sleep, but it was worth it.
The other thing that I did was go into his room 5 minutes or so before I knew he was going to wake up, and sit next to his cot and shhh. Just like when he was a newborn. Shhhd him through the 45-55 minute period when he was vulnerable to a wake-up. If he started stirring then I would pat him gently....again just like when he was a newborn. And, again this was absolute torture as it meant that I got very very little time to myself because the whole time he was sleeping it seemed I was next to his cot keeping him asleep. But, in the long run what was best for both he and I was that he slept...and if I needed to shhh him through to keep him asleep then it was worth it. Even today, I know know know that he is going to wake at 1hr20min, def OT for him, and I can't have that, DH is away for a few days, I am at my wits end with DS (teething, hugely into 2-1, OT, clock change, noisy neighbours....all have caused chaos happening at the same time). As much as I desperately need a break this morning I am going to go into his room at 1hr10minutes and sit there as long as I need to in order to keep DS asleep until he passes the "hump."
Anyway, just thought that if you are still looking beyond the possibility that this is OT or UT then they may be things you want to try.