Accumulator or cold water tank+pump

[John.g]

If you had 3 floors even with each only 2.5M high then a pressure of 0.75 bar will reach the top of the the third floor so you might argue that 1.0 bar (dynamic) would be more than adequate and it would, if you had > 22mm piping. 1 bar should theoretically flow 20 LPM to each floor (total 60 LPM) with 22mm piping but if 15mm is used then ~ 2.8 bar is required for the same flow rates. 1.6 bar minimum is still required at a flow rate of 10 LPM to each floor with 15mm piping so surely there is a requirement (and not from the water board(s)) to install adequately sized piping for each application.
1.0 bar dynamic will only give ~ 6 LPM to each floor with 15mm piping, assuming in all cases that a storage tank is being filled, also IMO the op will receive 0 LPM if the two floors below are flowing ~ 10 LPM each @ 1 bar dynamic.

 

[Chuck]

If you had 3 floors even with each only 2.5M high then a pressure of 0.75 bar will reach the top of the the third floor so you might argue that 1.0 bar (dynamic) would be more than adequate and it would,

During the periods when when the supply is '0 lpm' the water company is not 'supply[ing] water continuously', is it?

 

[John.g]

No, are they then obliged to either run a separate supply to the OP (the owner) or pay to upsize the existing (3 flat) piping? They also state that "where our communication pipe joins your supply pipe you should be able to fill a 2 litre bottle in less than approximately 13 seconds." so does this imply that they are only obliged to supply 10.9 LPM continuously?,

 

[Ant87]

Am I right in thinking that an unvented cylinder requires a minimum of 1.5bar working pressure and around 20l/min so even if SWW meet their minimum obligations of 0.7bar (at pavement level) and 10l/min then issues will still be present?

Something like a Stuart Turner Flomate might be an option which is an all in one stored water (break) pump set.
Dab E.sybox are extremely good units but might be overkill?

You are correct in saying that legally you can pump up to 12l/min directly from the mains with no break tank. Salamander HomeBoost is a direct to mains booster pump which will pump at 12l/min (1.6bar i think) however would still not be adequate for an unvented cylinder.

 

[Timmy D]

Can you explain why 12lpm is the max mate. It's been a long time since I did my relavent courses.

Water regs. As Chuck mentioned, to stop the mains supply running dry during peak periods.