expanded mesh

Hi all! I am currently building a 2 story extension. It’s a cavity wall, with 7n dense blocks outer leaf and a 3.6n aerated blocks for the inner leaf. What are peoples thoughts on expanded galvanised mesh? Do I need to use it in the blockwork? Is it vital? Is it overkill? Should I use it on...

I have a full room height vertical crack in a conc. block inner leaf that I may stitch with helical rods, it has a sand/cement render & skim. What width mesh (expamet) would I put over the crack before new render (and skim)? Presumably this wouldn't need to be stainless mesh as it's not...