Gents thanks for the replies, in particular Stephen your detailed one.

The supplier sent me a replacement unit I tried today ..and ... same noise.

Guitar is a Yamaha Pacifica 112J, The buzz is there without the guitar plugged in so took it as the guitar was not a factor.

The Boss katana mini amp's knobs run from 7 oclock to 5 oclock positions, & there are 3 settings clean, crunch, brown.

With volume & gain at midway (12 oclock) with the guitar plugged in, on clean in there is a persistent buzz , tolerable. I usually have it with gain at 9 oclock for practice.

With guitar plugged in, on brown the buzzing is annoying after a minute so I switch back to clean.

With guitar plugged in, on crunch for me the buzz makes it unusable.

So I have a budget £70 (~$88) amp & it feels like crunch & brown are unusable. Ok budget but surely all features should be useable.

I'm a beginner to the world of guitars/gear so maybe missing something in understanding.

Mike