Central heating system under construction, incl 200L direct thermal store,
contains about 270-300L of water - quite a bit more than average, I would
think.

Would the standard 4 gallon F&E tank be adequate, or should I consider a
bigger tank - i.e. would the 4G tank be subject to frequent overflow
episodes due to normal expansion/contraction?. If so is the next size up 25
gallons?.

Thanks,
Egremont.