Yeah, whatever
@Puddles I have recently learned to use the tool, having needed to take a cassette off - had freewheels before, so different tool. I knew what the chain whip was for, but did not know how to actually use it.
My friendly LBS
lady 
bike mechanic explained it all, and let me remove the cassette myself.
Discovered also that the last 3 small cogs of my cassette are detachable, so in theory you could buy replacements without buying a whole new cassette, alas the mechanic says nobody sells those cogs separately, so the reason why they can be removed must be an engineering/design one.