how can I increase low water pressure on 15 year old Potterton Lynx?

Your central heating system is a closed system, and presumably minor leaks have reduced the pressure. Somewhere there will be a valve connecting the mains water to the central heating system. With the heating off, open the water valve and watch the pressure gauge climb, turn the valve off when it's at the ideal pressure. On an older system, might be better at about 2 bar, and check pressure regularly thereafter.

david
December 2007