I'm about to start an MSc in September, which is a bit of a surprise as I never thought I'd get this far! But what I have no clue about is how a Master's is graded, it is pass, merit and distinction, pass and fail, 1st, 2.1, 2.2 etc etc?

You can pass or get distinction. Some universities have a merit. To pass you need 50, to get a merit if they have them you need 60 and 70 is a distinction. I hope this helps

Research masters tend to be just Pass or Fail. (Pass with minor corrections or pass with major corrections are possible too.)

Right, mine will be a bit of taught, and a bit of research!

It will then work: pass, (maybe merit) and distinction.

They are usually pass/merit/distinction, unless it's a "professional" masters that converts an existing degree into something else (like law or possibly psychology) - some of these may be graded 1st, 2.1 etc, but very rarely.

Hope it goes well for you and well done on getting your place!