Check the mains cold water feed valve is fully on. It only turns 90' from on to off.

It's unlikely changing the diaphram would effect DHW flow rate. There's a filter built into the cold mains feed to the boiler, I once found one full of sand, might be worth checking there before all else. Turn off the valve on the cold feed or use the stopcock. Have a bowl handy to catch any residual water and undo the (from memory) chrome nut on the front of the valve, there you will see the wire filter.