Thanks to all. The suggested script worked after I removed the bit:-
--getiplayer /home/don/Desktop/get_iplayer-2.80/get_iplayer
With that bit included, firefox gave message 'could not contact localhost:1935'. I looked in my /get_iplayer-2.80 folder and could not see the item get_iplayer therein (actually I found it in /usr/bin), so I tried without it and it worked. I'm guessing that, since I installed get_iplayer separately from the PVR program, the get_iplayer.cgi script is taking care of the launch of get_iplayer somehow. I'd appreciate a confirmation of that from a scripting expert
.
Also, I changed 'new-window' to 'new-tab' and that seems to work too, whether or not firefox is already running.
Many thanks again.