Vaillant Eco Pro 28 - Error flickering between 0.3 & F.22?

Hello,

I ran the hot water tap this morning to wash the dishes, it produced it's usual hot running water. About 15 minutes later I ran the hot tap again for something else and the water was stone cold!!

Had a look at the boiler and the red light indicator was illuminated, together with a reading flickering between 0.3 & F.22.

Please can someone shed some advice on this?

Thank you.