RainX Bodge kit from Asda works a treat!
At "birth", bullseye & crack, 200mm, then grew another 25mm getting home.
Spread another 25mm going to Asda the next day, and spread another bloody 25mm going back home.
Park in shade.
Remove any trim that would shade the cracked glass from direct sunlight.
Enter the magic glue.
Put tiny amount of resin at the downhill end of crack, and watch it wick it's way in.
Keep teasing a tiny puddle on the surface, but careful not to overtake the glue wicking it's way through the crack.
If the puddle goes too fast, air pockets will be left in the crack. So don't go too fast.
When you run out of crack, smear a little extra along it and then cover with the clear plastic sheets that fell out all over the place when you opened the packet.
Move car into full sun and leave.
Half hour on sunny day. A way lot longer on a cloudy day.
Why don't I just say "follow the enclosed instructions"?
& watch you tube.
The stuff works, the crack has gone from 1mm per mile to zero.
From now on, I will be keeping a tube of this stuff in the glovebox, as the sooner it's applied the better.
The youtube videos have the professionals drilling & punching crack stopping holes in the outer layer of glass. I am not a professional, so I chickened out.
But there are good engineering reasons for doing this.
https://en.wikipedia.org/wiki/SS_Schenectady