It’ll prob be the pressure chamber mate. You may find it all gunged up and backing up into the pressure switch itself causing the machine to think it’s full when it isn’t.
Best way to check is to watch it fill with the left hand side panel off. The heat exchanger should fill completely then totally empty via the pressure chamber. Watch to see if you get ‘backing up’ in the pressure chamber.
The part number for the pressure chamber should be 497570 – That’s if the ENr number you’ve posted is wrong and i’m presuming it is SGV57T03GB/08. I believe that they can be dismantled and cleaned out. In my position though , i tend to replace them as it’s mainly under warranty/insurance jobs.