Hi,
We've just started stocking a new 60 litre pressurised storage vessel, which can be filled from an ordinary household tap via a demin column. It's so new, it isn't yet on our website. You can fill it from your home tap, or if you have access to a tap at this 3 storey job, and the mains pressure is high enough to reach the third storey windows, it could be a very simple solution to your problem, with no need for battery, pump, RO or any other tank.
The total cost of the setup, including pressure vessel, with pressure gauge, and Hozelock connections in & out, 15M recoil hose, and 10 litre demin column, full of new virgin mixed bed resin, is very reasonable. It can be supplied trolley-mounted or not, to suit you.
Please ring me on 01704 541578 if you wish to discuss it further.
Hope we can help
Yours, Andrew