Oh wow I feel slightly embarrassed but ultimately relieved.
So after scouring every parts website and unable to locate a part I revisited the factory ecu terminals I have. As 93Clubman said above, all my research showed other models used the same terminals as the ecu but these look different… well…
The clockspring connector terminal only has 1 vertical slide tab and the NB2 terminal has 2. The overall body is 2 or so longer so I dismissed it as incompatible.
After looking at the white connector last night under my magnifier it actually has 2 recesses for 2 slide tabs!!
So I crimped a wire to an NB terminal and it half went in but then felt tight, bulging the white plastic slightly so stopped. Looking under my magnifier and confirming with calipers the wire crimp is ever so slightly narrower on the original terminal compared to the ECU terminal crimp, so I stepped down a size on my crimper, just slightly narrower than what’s required for the ECU plugs and Lo and behold - it slid straight in and clicked into place!
The longer terminal doesn’t stick out the back of the plug at all and the receiver end is exactly the same size.
Had the right parts all along -
TE Connectivity 316836-1 