Slippy ( by name slippy by nature 
) you are not doing yourself any favors.
Things come back on you as you are finding out to your cost.
You say she's a bit mad, her dear old friends are looking out for her by the sounds of things. Then what does that make you ? an opportunist taking advantage ( possibly ).
Whether anybody has seen you producing pure water there or not isn't the issue here. Do you really want to be working under the radar that way, knowing what you are doing is without her consent or worried about prying eyes ?
I don't mean to sound harsh and any digs are meant with humor, but do things right.
I've been in a similar situation re getting a supply of water in order to work. Its difficult yes but not impossible. I had to run an ro out the window of my first floor flat to my street parked van for 7 + years. The only reason I don't anymore is because parking has become increasingly difficult and began dictating my working day and free time. I sourced a supply of ro water from another windy that I pay for.
Can you not do the same ?
Trying to save a few quid or obtaining water fraudulently is only going to end badly and leave you high and dry ( at the least ) without the means of being able to work.
How did you run an RO from kitchen? Was it in the kitchen. I guess a double DI would be quicker. Did you have any problems with neighbours. Something I am thinking of.
Anyway, apparently the German lady has decided that the deal is to water her plants every two days for free in payment for free storage of so many gardening and window cleaning equipment. The subject of using her water or re-charging pumps with her electric socket weren't mentioned. Reluctant to ask it. But to be honest if you struck up a deal of using her water for free gardening or a payment it could be very beneficial. How would you approach the subject with her? Or maybe see if it can be worked out from own flat first then if that fails ask.
As she's completely bonkers it might have to be negotiated with her various old biddy 'friends' 2 who are just trying to get her £600,000 house when she dies off her.
Oh yeah and she met Hitler in person when she was a kid. No lie.
I put a self tapping washing machine tap on to the mains supply in my hallway where it enters my flat ( after the stop cock ! ).
Mounted the ro high on the wall so it was not in the way.
The waste hose went out the back and in to the drain down pipe, the pure went under floor boards and out the bay window at the front. When I needed to produce pure I'd connect a reel of hose to it and lower it down to street level and connect up to a hose outlet at back of my van. My neighbours were ok with it. I always cleaned their windows when I did my own to keep them sweet.
You could have offered to rent the space off her, to include water and power, money talks. She owes you nothing, you on the other hand ........
She's elderly, could drop down dead tomorrow. Hardly a beneficial situation to be in. Sort something permanent out for yourself that you have full control over.