Well - I *did* end up hacking the backup file, as the UI does not allow this.
The backup file is a line-by-line base64-encoded file (no: they didn't encode the file as a whole, but encoded each line individually !??), which contains something that attempts to resemble XML, where the XML data for port forwarding is a weird ASN notation (which makes the file invalid XML, so they can't have used the normal XML parsers either). In short, the file format itself with worthy of www.thedailywtf.com .... it will even break if your admin password has weird characters like semicolons or bits of XML in it!!
I managed to write a couple of shell scripts which will decode / encode the files and generate the config I want - and it loads OK - if somebody else has the same problem, I'm happy to share the scripts upon request : drop me an email at ############.
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