Basically you need to leave the flats in a condition that they just need a sparkle clean i.e vac dust & polish.
Windows are key
Scrape off any tapes, plaster spills, clean round hinges and inside window jams and then clean windows inside out. ( hence a unger scraper is usually the best). However must be careful not to scratch paint or plastics.
Remove plaster spills off floor and skirtings, vac and wipe all tops and surfaces. Floor scraper maybe required.
Vac and wipe inside cupboards and units, don't forget to wipe inside electrical cupboards and around airing cupboards.
Clean/blade down tiles in kitchen & bathroom
Equipment rq.
Scrapers
Squeegies
Mops
Buckets
Soft and hard brush
Vac - make sure it is regualrly cleaned out as builders dust wrecks them
Rags - buy 10kg pack of rags such as laundered towels
Polish,Multipurpose cleaner,degreaser,spirits