Just an update that might be of interest.
One of the links above suggested being bit a 'crafty' and avoid the queue at the money exchange in the arrivals hall and nip upstairs to the departure hall where the exchange booths were likely to be much quieter. We took this advice, they were certainly very quiet, but all three booths refused point blank (and most resolutely) to change money into CUC's. They were only interested in changing from CUC's to something else. Went back down to the arrivals hall and joined the orderly 10 deep queue feeding the 3 exchange booths. All done and out of the airport in less than 10 minutes so that was good.
As others have said, cash is clearly king. We were actually on a small cruise ship and called at two ports that had money exchange faculties at the port - all worked ok.
When we eventually decided to change our remaining CUC's back to 'sensible' money the exchange clerk laughed and scoffed when I suggested I would like £ or Euros. He said he could only do US$. We gave him 68 CUC's and received $65 in exchange which I thought was pretty good?
Fascinating place.