Loss of pressure?

I have a Worcester 28 cdi and functions normally...or so I have believed. When it was installed, the installer, who was corgi registered, demonsrated how to go through the topping up process when the needle drops a couple of divisions on the pressure gauge. It was never explained how often this is likely to occur. I top up the pressure on average about every 3-4 days. The pressure is set at 1.5 on the pressure indicator and has been known to run with the pressure as low as 1.0

There are no signs of water escaping from the connections at the radiators.

To measure the loss of pressure I recently did a test, the result of which I find alarming.

Having set the gauge to 1.5, I then bled a rad and found that the loss of 0.5 pressure on the gauge equates to something like 1 pint of water, which to me seems a lot.

To go from 1.5 to 1.0 takes about a week which also means the inhibiter becomes weaker each time topping up takes place.

Thank you