Severly cracked rear screen, any tips to prolong?

You can get window tint megacheap off ebay (£4 for enough to do both sides) and I'm sure that would hold it in one piece for the time being

You won't stop it getting worse unfortunately Rob, if it's cracked not crystalised that would suggest that it's a laminate not toughened. Has it cracked the inside or the outside layer or both? If it's laminate the plastic layer in the middle will stop it leaking/falling in/exploding next to bus stops full of nuns carrying kittehs.
Mates screen on his escort van cracked right across the bottom after a stone got kicked up by a lorry on the way back from the MOT, he only changed it for the MOT the following year because he had to. We used to draw a mark at the end of each of the cracks radiating out from the impact with a sharpie whilst sat in traffic on the way home from work, just to see how it was progressing. All the cracks stayed in the bottom 3" of screen so visibility was never impaired.
Tint or laminate would hold it together if it did decide to let go but to put the stuff on you'd need to press on the inside of a concave window, which in theory is its weakness. Even on the outside I'd be scared I was going to make it worse. I've cracked a couple of Ford 'screens in the past just putting the stupid stick on interior mirrors back on.

clear sticky back plastic ?? temp repair for now

if you want to stop a crack spreading there is only one way
you need to drill a small hole at the end of the crack as a crack cant jump a hole.