Each flat uses approx 2,500 units of electric per year.
Most of UK s electricity supply at night comes from Non-Renewables (beside Windpower)nstall x number of storage batteries, charge them using lower rate electric,
I strongly recommend to install PV , if possible in the region of about 10 kW
You can do the sums , but at one stage in life , the installation will be paid of , with the help of FiT and the sold Units to the 3 flat renters
Once is paid of , you than have the gain of say 10 years or more free units that you still earn FiT and rent , say think about a 25 year life span ( but include battery replacement and electronic gear failures)
Also i would also consider a big lead acid battery
Billi