OK. I finally got around to calling Iberia Plus on skype, complete with some angry shouting in Spanish, partially to be understood, partially as complaint. Here's my experience.

1. Call +34 901 201 241. As always, Spanish is better than English with IB+.
2. Mention that your account might be blocked because you can't book anything, combine Avios, etc.
3. I was told that there was a "Campaign to Update Data for Certain Customers". Cue sarcastic comment by me about not communicating said campaign to members affected.
4. I was given the option of running through the data IB+ holds on me. If it matches, they would send me an email to reset my PIN. Everything can be found in the "your profile" section of your IB+ account, so log in first to see what IB+ have on you...
5. Name, passport number, birthday, email address on file.
6. More arguing when she said I gave the wrong email address. Turns out she misunderstood me.
7. Email arrived in my inbox immediately. PIN expires within 24 hours, and on log-in I was prompted to change it again.
8. If your answers don't match, IB+ said they would send a form to be filled in, that needed to be returned to [email protected] along with a photocopy of ID.

Combine my Avios now working and I can now get full quotes for award tickets. Would put a smiley face, but loyalty programmes shouldn't really block accounts for several weeks and make no effort to communicate.