Really need some advice please :)

On the contrary, my understanding was that it is safe to compost dead weeds after treating them with Roundup because it doesn't persist (unlike Aminopyralid and Clopyralid)

I asked because I thought you might have a different reason - e.g. seeds on the (killed) weeds, or parts of roots, that might survive etc.

Directions for use instructions for Roundup from Monsanto's site:
www.roundup-garden.com/files/download/80‎

"Dead or dying weeds can be dug in or composted without harming future crops."