Peter Cross replaced my rear sweeps last year (all the way up to the rh rear window on one side). Did a fantastic job. I'd recommend him any day...

Just ordering new rear sweeps now - finally!

Thing is, what if the cracking is caused by the body flexing I don't want it to happen again

Now, I know my car's sitting on a bendy-flexi 2cv City chassis (which, I haven't had issues with in the past) and the front floors are going which probably doesn't really help, and I don't like the idea of the body of my car flopping about.

What can I do to stop the rear sweeps cracking again?

no rear sweeps, no cracking.

That had occured to me

The extracted rear sweep.
Some silly person had welded a patch over rusty metal, bending the old damaged sweep out of the way behind the new patch