many thanks, definitely i will give the pressure calculation a go.

For a sealed cylinder, the pressure is the same as mains, no matter where the cylinder is. And if too many taps are open, it will be the same no matter where the cylinder is.

Thats a fair comment when no tap is open.  There will however be a dynamic loss once a tap is open, which I think is the sum of the loss in the pipe work leading to the cylinder and the loss in the pipework from cylinder to outlet (so when a tap is open the pressure in the cylinder reduces).   I think thats the case anyway!

This looks like a good calculator to use.

But if you are able to work out all the kinks of the pipe like @jamespa suggests, great.

Assuming you have good flow and pressure in the mains and normal pipework, I would not worry where the cylinder goes.

edit:

if there’s limited flow or pressure, the length of pipe runs can make a difference…

I've offered that. however it is very disruptive, will have to remove the wooden floor to repipe and to install discharge pipe(D2).

for me the loft is ideal place, apart from the work needed to be done prior to the installation(structural and joinery)

also the concern of losing water pressure and velocity 

OK fair enough, just asking.

Its possible to do pressure loss/flow rate calculations if you know the pipe length and roughly the number of bends see eg: https://johncantor.uk/calculators/flow-rate-and-pressure-drop-simulator/

Alternatively if one of your current outlets is about the same distance away you can measure the flow rate there.  

3.8 bar static, 3.2 bar dynamic, the new cylinder will be in the loft, the old system is vented.

Do I gather you have an existing DHW cylinder - if so does it really need replacing anyway?

the old tank is small and old for 5 adults

Nothing wrong in principle with putting the cylinder in the loft albeit not ideal, but looking at the floorplan I would have thought there might be better locations, eg the store next to the bathroom on the ground floor.

I've offered that. however it is very disruptive, will have to remove the wooden floor to repipe and to install discharge pipe(D2).

for me the loft is ideal place, apart from the work needed to be done prior to the installation(structural and joinery)

also the concern of losing water pressure and velocity 

Upgrading the existing 15 mm mains cold water supply is not included within the scope of the heat pump installation.

Whats your mains pressure like, what sort of flow do you get at the cold water tap furthest from the incomer.  If you have good mains pressure and/or the tank location is close to the incoming mains the upgrade wont be necessary because you will get good throughput anyway. 

Do I gather you have an existing DHW cylinder - if so does it really need replacing anyway?

Nothing wrong in principle with putting the cylinder in the loft albeit not ideal, but looking at the floorplan I would have thought there might be better locations, eg the store next to the bathroom on the ground floor.  Obviously I dont know what you use the space for!  Are you comfortable that the loft is the best place.  Incidentally it could be smaller at the cost of a bit of efficiency (heat less water to a higher temperature) if that made a difference to practicality.

Our home is a bit different from the usual. We have a pressurized cold water cylinder with a in&out 22mm pipe. This means that, no matter what the size of the pipe taking water into the home, multiple 15mm pipes going out of that main 22mm can have maximum flow (until that tank is empty). 

If your mains are 15mm (unless you have anything similar to what I describe installed), having 22mm will not make much difference. I say much since the wide the pipe, the less pressure drops (but I believe this is not significantly different). 

• Vaillant sensoCOMFORT single-zone smart thermostat
• Internet gateway
• Outdoor weather sensor

• In case your hot water pipework is over 15mm, it makes sense to get the pipes in and out of the hot water cylinder as wide as possible - so nobody sees reduced flow in a shower when someone else flushes a toilet or opens a tap. That´s assuming the cost and disruption is acceptable.  

  is the main cold water supply, my worry how far i need to go to replace the 15mm, the cylinder will be in the loft and 15mm starts from under the house.

• In case they include up to 60% rad changes in all their quotes, it makes absolute sense to try to have as low flow temperature as possible. This can only improve  SCOP+ youget new rads (even if the new ones may be larger and restrict use of rooms). 

   

• so far they only offered 2 replacement rads, i guess they trying to save cost, hence 50 degree flow

...

is it a haggling process with eon since they can swap up to 60% of rads

Looks like a very detailed and well comprehensive heat loss calculation with all the related statements. 

In case your hot water pipework is over 15mm, it makes sense to get the pipes in and out of the hot water cylinder as wide as possible - so nobody sees reduced flow in a shower when someone else flushes a toilet or opens a tap. That´s assuming the cost and disruption is acceptable.  

In case they include up to 60% rad changes in all their quotes, it makes absolute sense to try to have as low flow temperature as possible. This can only improve  SCOP+ youget new rads (even if the new ones may be larger and restrict use of rooms). 

Being a Vaillant they are probably including Sensocontrol but may not have included the gateway (required to use the app).