After more trial and error I have discovered that I can forward any port from 10000 to 65535. That solves some issues, but with a lot of inconvenience in having to change the links on my external devices.
But I can't put any port lower than 10000 in the Local Port box. I can put any port (including numbers below 10000) in the external port box - but that isn't any use (strangely the other way round would make sense, and enable me to solve the problem).
The error message I get is "The local ports between 21 and 21 on IP 192.168.0.20 have been added as virtual server already, please try another one." The message is in red. Needless to say I haven't already added the 21 port.
I posted too soon, the first test set up worked, but the real rule I want to set up did not work.
The position I have now is that I can set up the rule for 192.168.0.20 (did this to test), but when I try to set up the rule on device 192.168.0.10 I get the error. This is for any port under 10000 (which was the problem I first posted above).