Low flow on Mains Medway water heater?

I have tried descaling the pipe work of the heat exchanger, and when connected to a hose the flow rate seems fine. However, when back together the flow is very slow, and the gas jets don't really get going. The mains water pressure is low, but not that bad. However it has got worse over the last few years. Is there an adjustment that can be made? Is there anything other than heat exchanger that can get clogged up. Thanks for any advice. Main Medway, serial no. 55-54.