Barry,
Looks like you have the same plans as us! Will you PM me with any further information you find? Luckily this year we haven't booked our flight with XL like we usually do.
We've booked overnight in Rhodes for the 17th and possibly the 18th as we want to go to Nisyros then onto Tilos, then Symi. We would much rather go to Nisyros on the 18th even if it means going via Kos. I'm checking those ferry timetables daily but at the minute it looks like its The Diagoras on Friday at 1pm.
Looks like we may bump into you on Nisyros!