So, this is the theory:
and after much hunting, grunting, being pissed off at total lack of consistency of describing springs, and the damn insistence of using bloomin inches for everything, I got far enough down the comments and found a part number for a slightly stronger spring that is more compatible with OEM struts (too weak and it doesn't jump up enough to grab, stronger spring or doubling up needed). Anyway, eventually tracked down this on order from the US:
Prime-Line 7/16 in. Dia. x 1-7/8 in. L Extension Spring 2 pk, SP 9605 49793096050 | eBay
£4 including delivery from the US! But the hassle of ID'ing it still pains me.
I've ordered these:
https://www.amazon.co.uk/gp/product/B07WYJ58NF/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1
TBH, I have my doubts at ~£10, but will see! If these are up to a million miles why didn't tesla just install them in the first place? I'm assuming cheap rubbish, but for the price I can't bring myself to cancel the order. Will be very wary when fitting.