Too Much Coffee

Because we have very soft water I assumed I would never have to treat the internals of my Silvia to any cleaning procedure. Yesterday I didn't like what I was tasting so I immediately did a press-pot brew to compare. Because I'm quite meticulous with the head I had a look in the holding tank, I saw a vague slimy deposit so gave it clean.

Does this mean I need to treat the inside of my boiler together with the associated pipes with a cleaning routine and what would I use ?

I take the head off once a week, give it good scrub with soapy water and that's it.

I do not clean my boiler, but once a week use Puly Caff in a blind filter. This washes out a lot of brown liquid, even after prior cleaning of the dispersion head.

Whether your boiler needs cleaning or not is questionable. You might just fill it and empty it and see what the water looks and tastes like. Scale buildup doesn't necessarily cause a taste difference, but buildup of algae in the tank definitely will. The tank needs a good cleaning if there is a slimy buildup in it. Hot water and dish-washing detergent will do the trick, not descaler or espresso machine cleaner (Puly Caff).

A complete change of water in the boiler will then get rid of any unpleasant taste from there.