It does sound as tho' your combi is being fed directly from the mains (which is normal), and also that your mains is being shared by the flat below (not so good).
As far as I understand,
all the WB will do is to confirm that the pressure and flow at
ground level is compliant. What the flats then suffer is up to them. If there had been 4 different stopcocks/meters out on the pavement - one for each flat - then I don't think you would have any issues, I'm pretty sure. I don't know if there's any obligation on the WB to change this - eg provide
individual supplies?
Some things are weird, tho'. You almost certainly have your taps running directly from the mains but you also mention that there are storage tanks in the loft? Which flats do they supply?
Another weird thing - you also said "
I spoke to the plumber fitting it and he checked out their fittings and said it was all fine, also turned the water pressure up a bit, made a very slight difference but didn't change anything with the temperature/pressure drops."
I suggest you ask this plumber what he actually did to 'turn the pressure up' because there's no way that a plumber working in the flat below you should be able to alter the supply to
your flat! It almost suggests that the cold rising mains passes through the flat below first, and they have 'control' over it before it continues to you! If they have a pressure reducing valve fitted to their incoming mains (ie - is
that what the plumber 'adjusted'?) then it shouldn't be allowed to affect you, so you'd need to insist that
your mains comes off that main rising main
before any such valve or control. (That's all dependent on the actual situation.)
What to do? Blimey...
I guess first check the obvious - like fully-open/faulty stopcocks as mentioned before.
Priority, I think, ask that plumber what he did to increase the pressure; where is this control, how come it affects you, basically - what the hell is going on?!
The solution to your problem comes down to what the cause is. You cannot
necessarily compare your flat to the others in the block or street because you don't know what systems they are using. For instance, it might be that everyone else is using the cold storage tanks in the loft and therefore have no issues! Perhaps you are the only one still directly on the mains (apart from the flat below you who 'steals' all your pressure...)
If there
isn't an obvious 'cause' of this that can be properly resolved (eg by adding a direct mains supply from the ground to your flat by-passing everyone else, or finding an 'issue' in your neighbour's flat that can be fixed) then you may be looking at coming up with your own 'cures'. The most reliable, I think, would be for you to also use a stored water tank in t'loft rather than relying on the unreliable mains powered water. Since you have a combi boiler, tho', this water tank would need
pumping down to your flat to enable an adequate enough flow to your combi boiler.
There are other solutions such as mains booster units which are a bit like a stored water tank except it does so under pressure (something like
Stuart Turner Flomate Mains Boost Extra 100 Low Water Pressure Pump 46635 . This will pump up the mains supply and store it under pressure for you to then use in your flat - at whatever rate you want (so no further pumps needed). It automatically keeps itself topped up. If you use up the whole tank 'cos you have a long shower, then it'll carry on pumping up the mains to keep giving you at least 12 litres per minutes. You can get them with larger tanks too.
You'd need to confirm with the WB whether you are allowed to fit such a booster on your mains - most allow out, but some possibly don't.
But, first ask
that plumber what he did in the flat
below yours to boost
your pressure a wee bit - 'cos that sounds wrong.
