Vaillant turboMAX plus 800 boiler with broken built-in filling loop

Hi there. I have a Vaillant turboMAX plus 800 series boiler with a built-in filling loop. The two brass knobs that allow water pressure regulation have been stripped so I can't now pressurise it. Can I get a replacement part for the built-in filling loop and have this fitted on site? Or is it possible to just leave the filling loop open and fit an external filling loop to allow pressure regulation that way? There seems room for this on the water supply pipes underneath. I'm trying to avoid replacing the boiler as it's old but reliable and otherwise in good nick. Please advise. Very grateful for any help you can offer. Jon S, Brighton