im also in a first floor flat!i think the 2 DI set up with tap water in the tank then di to brush is the best economical option.i think a 400 lt full tank should last me most days.wont have a problem if i need to top up though on the job so to speak as ive been cleaning some of my custys for 15 yrs and most are loyal and friendly and are not on meters!!but ill try not to as thats wasted time not cleaning!