They are simple. You have a check valve after the stop cock. After the CV have a tee to the accumulator. It does not have to be near the stop cock, with location anywhere along the cold line. Bingo!!! That is it. You can have a smaller accumulator just for the showers only. The piping has to be arranged to suit. Whatever the size of the accumulator, say 150 litres.they hold approx half of that. 75 to 100 litres is OK for a one bathroom flat that mainly uses showers. As the pressure drops after the check valve when water is drawn-off, the mains will still feed in. They are simple, non-electric, saving space. It is best you replace the stop cock with a full-bore cock.
I am helping a customer with the same problem as he is on the fourth floor and if the flats below use any water then he gets none.
There is a simple solution for him but I dont know if it applies to you. If you would like me to consider your situation then you will need to tell me when the building was converted ( if it was ) and when your lease commences and how long it is and where you are located.
In that case I am afraid that there is no easy solution for you that I am aware of.
Scottish law is significantly different fom English law although some parts have been modernised in more recent years.
Its a common problem in old properties but apart from asking your neighbours if they have any easy solutions I think that you will have to pay for your own independent supply from ground level.
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