Low Pressure Light Lit - Pressure Fine Though?

Hi I had to bleed one of my radiators today which lowered the pressure on my Glow Worm Swift Flow 100 combi boiler to almost zero so that the pressure light lit up.

I found that I had to open the valve on the filling loop. I did this until the pressure reached 1 and closed the valve again. The pressure now reads 1.5 when the unit is on but the light still flashes.

I also noticed that when the hot water is run - let's say in the shower - that the pressure slowly goes down but then slowly returns after the shower is turned off again. (not sure whether this is normal or not)

Anyhoo what do I need to do to sort this out?

Thanks for your help.