there's a tiny square open notch on the bulb-bass of the pot, you need to aim the contact cleaner into there and spray a tiny amount while rotating the knob, which will mean opening up the casing to get to it. Make sure there's no liquid on the circuits before turning back on. If it's still under warranty (unlikely since they haven't been produced for a while),
Marshall would do this, or replace the pot for you if you drop it off at a local Marshall dealer, and get them to deal with the returns number/collection etc.
Scratchy pots are irritating enough, but if the volume dipping leads to the level knob not working at all (still getting signal, but no control via the knob), then a factory reset will do the trick most of the time.
