Sudden low water pressure after no use in 3 months

Hope you Guys can help....

The scenario is ... After not being used for 3 months the water
pressure in the flat has dropped by 50% at least (it was fine before
and the assumption was that someone had turned the gate valve down)
Mains is fine but the rest (hot and cold) are very low.
Water is fed from a cold water tank in loft (2 floors up) and apart
from the gate valve at the output of the tank .. there is no other
valve/stop on the system (between tank and taps etc)
The gate valve shuts the water off when closed and feels "normal"
while opening and closing and the area isn't badly affected by
limescale so I assume that the pipes aren't furred up (not after not
being used for 3 months anyway)
I'm at a loss as to what could be causing this problem and would be
very gratefull for any suggestions.

Cheers

P.S Email address doesn't work to avoid spam