Usually, Masters courses are marked either as Fail, Pass or Distinction (sometimes, not even that). So, if you don't get a distinction, then an average of 45% may as well be 65%. In that case, the only way it matters is when it comes to getting a reference. So, if you don't think you're going to get a distinction overall, make sure you get one on your thesis, and are at least close to getting a distinction overall. A good reference from your thesis supervisor will then help quite a bit.