Loss of HW pressure

hello, having recently purchased my property 5 months ago, the phenominal HW pressure originaly deployed has droped dramatically, as if being served directly by the condensing boiler. On investigation i have found my system is ran via a systemate 2000 served by a potterton boiler. could do with a few starters to try and diagnose this problem. The only recent change i can think of was the rare occurance of topping up the F&E cistern, the recent owner suggested i shuttle the suppy valve to this cistern to meet demand but don't leave it permanatly open. I have often checked this cistern but never found it to demand water when the supply valve is open.