Hi Richard,
I'm walking the HRP this summer, and have found myself in a similar predicament to you, although there's only one last map I'm still chasing (cutting it quite fine, eh?!).
I got all of mine through www.mapkiosk.com - have you tried them? The only one I haven't been able to get a hold of is the Editorial Alpina set of 2 (Baztan-Bidasoa) which Joosten recommends. I'm still at a bit of loose end regarding what to do about this, but I think I will have to settle for what Ivo suggested above.
Do keep us posted and best of luck with your planning!
Alex