Just a further update on this , the chaps from here
https://aerolite.org.za/
came to install yesterday and they did a fabulous job. They also covered up the geysers (well re-did it) and insulated all the pipes too (I had done the warm water ones previously), They were clean, fast and professional . They also removed some roof tiles to get into a section that had a small crawl space to insulate - so very thorough. I was quoted 22k for 135mm aerolite for what I said was 280sqm. It ended up being 250sqm and total bill was just short of 20k.
Going to recommend aerolite and this company. We feel the difference already.