Practicing with the oddball keys (those that are rarely used hence I've always historically 'glanced' at the keyboard prior to using them) I found that the (my keyboard) oddball couple top left under escape, left of 1 are wrong. No idea how that occurred, but it did. This time I went straight in. /etc/X11/xorg.conf took me to /usr/share/X11/xkb/rules/base.lst, wherein I found gb in the ! layout section. Straight from there to /usr/share/X11/xkb/symbols/gb wherein I found

partial alphanumeric_keys
xkb_symbols "dvorak" {
    include "us(dvorak)"

    name[Group1]="United Kingdom - Dvorak";
   
    key <BKSL> { [ numbersign,	asciitilde	] };
    key <AE02> { [	    2,	quotedbl,  twosuperior,   NoSymbol	] };
    key <AE03> { [	    3,	sterling,  threesuperior, NoSymbol	] };
    key <AE04> { [	    4,	dollar,    EuroSign,      NoSymbol	] };
    key <LSGT> { [  backslash,	bar		] };
    key <AD01> { [ apostrophe,	at		] };
};
</AD01>

Wherein I added a line

 key <AE00> { [ grave,   notsign        ] };

Saved the change, logged out - and back in (how to restart the X server?) and tried it. My oddball key is now as per the sketch on my desk. I'm feeling smug!