I've finally got something to work. The issue is that the FTDI driver recommended by Moonlite on their site version 2.4.2 from www.ftdichip.com/Drivers/VCP.htm JUST WILL NOT LOAD on Catalina complaining of invalid signature. According to some other forums the loading can be allowed by bypassing Apple's integrity check in safe mode but this is for all software, not just this driver - not a workable option for me.

From the site above I downloaded a 'no longer supported' version 2.3 and this now loads. Not ideal but at least the focuser works now. If FTDI and Apple get their act together and release a supported (by Apple) version then this will have to do.

From System Report -> Software -> Extensions, we now have the following, whereas the newer version 2.4.2 was not loading:
In FTDIUSBSerialDriver:
Version: 2.3
Last Modified: 02/03/2015, 15:37
Bundle ID: com.FTDI.driver.FTDIUSBSerialDriver
Notarised: Yes
Loaded: Yes
Obtained from: Identified Developer
Kind: Intel
Architectures: x86_64
64-Bit (Intel): Yes
Location: /Library/Extensions/FTDIUSBSerialDriver.kext
Kext Version: 2.3
Load Address: 18446743521889628000
Loadable: Yes
Dependencies: Satisfied
Signed by: Developer ID Application: Future Technology Devices International Limited (658CPPCMJJ), Developer ID Certification Authority, Apple Root CA

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