We have a house with 8 bedrooms but we only occasionally use them all when the family come home and then there isn't enough HW. Can anyone please advise whether adding a pre-heat cylinder in series with the main cylinder is a good idea ?
If you have the space, put in another cylinder (in series with the existing if you want) and connect the primary coil to your heating system via another motorised valve. Fit immersion heaters as a backup if you want (if you have pv solar panels they may be a handy energy sink) but electricity per kWh is far more expensive than gas.
Assuming you have gas central heating the boiler will 99% of the time have enough spare capacity to heat the extra cylinder.
If you have many bathrooms only used occasionally you could consider plumbing your new cylinder so it only supplies the occasionally used rooms...
EDIT You may need to increase your header tank capacity, depends how quickly your existing one replenishes.