So, after a few days I (in desperation!) reached out to the great people at Protectli who sold me the router hardware in the first place. Their support was awesome. Thanks protectli!
Someone there set up a working example and then exported the xml description (diagnostics > backup & restore) so that I had something to compare against, line-by-line. After an hour or so combing through mine and theirs, I discovered I had wrongly configured a static route early in my experimenting with pfsense, which was disrupting access on the WAN interface to the gui. Once removed (and after a reboot) access via WAN was restored. "Problem" resolved.
Many thanks @Bob-Dig and @johnpoz for helping me through the initial stages patiently; much appreciated.