Pressure in my Ariston Combi Boiler drops when switched off?

I recently purchased a new flat, which has an Ariston Combi boiler (installed last year along with new heating system by previous owner). If I switch the boiler off (Eg on timer) the pressure gradually starts to drop. If left approx 8 hours or more, this drops to a level where the boiler fails to switch back on, error message appears and pressure needs to be corrected by opening water inlet valve briefly.

If the boiler is switched back on within approx 4-6 hours, the pressure is adequate to restart and gradually comes back up to a 'normal' level.

Any ideas? The person who installed the system cannot schedule a visit to me for a month, so keen to get an idea of whether this can wait or should be looked at sooner. Plus it is costly to programme it to come on every 4-6 hours to avoid this issue. Appreciate any advice - thanks!