Hello,
Over the past month or so we have noticed that water pressure will periodically fall for a second or so then return to normal; the water flow does not stop completely. This is happening every 10-20 seconds, with both hot and cold water and from all taps/showers in our flat. It happens even when only one tap is in use. We have an electric Megaflo unvented cylinder. There has been no modification to the plumbing system either within our flat or within the block as a whole, since it was built 3y ago. Any ideas?
Thank you

Research indicates this is a UK system. "within the block as a whole"
Have you checked with and neighbors to see if they are experiencing the same issue? If not, it would appear the problem is isolated to your flat. Do you have a booster pump? Looks like a typical HW heater. In the US, none of similar units are vented, only protected by safety overload release valves and expansion tanks.

This usually happens when washing machine kicks in undersized plumbing system, or even when somebody flushes toilet. Also, if your irrigation system is connected to your house / flat plumbing, such water volume reduction is noticeable.

Start paying attention to what is operating the exact same time you notice drop in water pressure. As mentioned above, see if everybody in your building experiences same problem

Let us know what you find out

Back to you

Milo