Puma 100e - increase in pressure when CH on?

Increased the pressure in relative's Puma 100e following being told the Low Pressure Warning light was lit. Likely cause was Water Co. engineers when fitting water meter. I found the "top up" pipe and valves and increased pressure to 1 bar when cold (as instructed by label on side of casing). Light went out and boiler started OK. After approx 20 minutes I noticed the meter is climbing. When it reached 2.5 bar I switched the boiler off because the house is unoccupied, the idea was to have the CH on twice a day during this cold spell. I checked that both the valves on the top-up piping were shut off completely (one simple on/off valve and the second was a radiator type valve). Any advice please?