How to heat water in electric only flat?

I know you say flat, so I don't know if you're renting but: Save yourself the effort and have it replaced with a instant hot water heater. It has nothing to do with being electric-only in itself. I live in an electric-only house, but it came with a very poor and outdated immersion heater setup that's on a three-rate tariff. My hot water only turns on overnight in very specific hours and it's essentially useless. I would spend a fortune heating water to have a single warm wash/shower in the morning... and then it'll sit cooling all day until I get home from work. And I'd have to press a "Boost" button on my meter to get it to warm up again if I wanted another shower before bed, for example. It's just not worth the money or effort. I'm replacing them with an instant hot-water heater under the sink (that can run two sinks but not at the same time, but that hardly matters) and an electric heating power shower when I get the money. The old fashioned shite that runs on immersion heaters, dual ("Economy 7") and triple-rate meters ("storage heating"), etc. are designed for an era where someone was always at home during the day (e.g. a housewife) and waste so much energy at the wrong times just in case you want to use them. A guy is literally coming out to me today to change my meter for a more normal smart meter so I can start getting rid of this crap. I'd far rather have a tiny thing under the sink and hot water whenever I want, and only pay to heat what I actually use, and a shower that's instant-hot and again only heats what I actually use, and then reclaim a huge cupboard full of nothing but plumbing and tanks, and reclaim my loftspace which has a large header tank that has obviously leaked once before. The previous guy who lived here used TWO AND A HALF TIMES as much energy as I use... and that's exclusively the poor heating, poor water heating and things like having 800W of bulbs in a single fitting in the kitchen. Even with the heating, although a convector heater isn't the cheapest way to heat a house, it's still far cheaper than pumping energy into bricks overnight, not being around during the day to feel any benefit, and then it being cold by the evening and requiring a boost. Instead I bought a "smart" convector heater - that makes sure the house never goes cold enough to condense water on the windows, etc. and I turn it on on my way home so it's warm to come home to. It uses half what the storage heating ever used. The real irony is that my storage / water heating rates are actually MORE than my peak electricity rates. So I save absolutely nothing by using them whatsoever. There are probably hundreds of thousands of old people still stuck on that old system and don't even realise that it's costing them money to be more hassle and leave them colder than if they just bought an electric heater.