In case anyone wandering why I've gone quite it's because I'm reviewing words I've learnt until I get a 100% hit rate when trying to remember their japanese counterparts
Don't do that. It's a well known fact of SRS (spaced repetition system) memorization that a ratio of around 80% success allows you to memorize the most information. Forgetting is natural, and trying to memorize perfectly means that you end up learning less in total.
That is to say, if you practice twice as many words at 80% success rate, then you'll have learned 1.6x more words than trying to hit 100% success rate.
If I understand correctly, memrise is basically an SRS flashcard system with a lot of multimedia options and premade flashcard decks (which they call 'lessons', but don't misunderstand, they are decks of digital flashcards.)
For your reference:
Spaced repetition - Wikipedia
I also use a spaced repetition system (Anki), and so does Mike Cash, although I'm constantly forgetting the name of his favorite system. So, first of all... they are great at assisting you in learning and retaining vocabulary, but they really don't do anything beyond that. That is, you won't learn much in the way of grammar from them. If you have a lot of full-sentence cards (or questions or whatever your app calls them), you might learn some common fixed expressions and reinforce grammar that you already understand, which is also useful, but it's hardly everything.
The other thing is, that while 80% is the theoretical ideal learning rate, a lot of people (including me) find it really frustrating to fail that often. I aim for a 90% success rate in my reviews, because while it may not be the mathematical maximum, it feels a lot better and it's a lot easier and more enjoyable to review when you're answering correctly 90% of the time. (Maybe this is an artifact of school grades where 90% is often the cutoff for top marks, or maybe it has to do with the way we are emotionally hurt more by failure than we are gratified by success. Or maybe it's just me... many people do run at a strict 80% rate for mathematically ideal learning.)
I don't use memrise so I can't say for certain, but common ways of manipulating success rates are 'difficulty' levels in correct answers (wrong / easy / normal/ hard), settings that adjust the review period scaling, and rates at which new cards are added. Oh, and 'extra' reviews, or 'cram' modes, which review in addition to the set timings, which you should basically never use unless you're desperately pushing to pass an upcoming test. They're terrible for your long term learning efficiency.
The defaults in most quality apps (and memrise has a good reputation) are pretty good and you shouldn't screw with them to try to hit an unrealistic '100% success rate'.
I did watch the video and I don't know that I feel good about that course you're using. I like the A/V cards with their full sentences, but you also had a bunch of romaji only cards, and then you had a bunch of cards that seemed to be full kanji. You should really be using all-kana right now, and a slowly adding in kanji.
Romaji is fine for starting out, but you should be done with it by 2 months into your studying, and everything should be kana or kana/kanji mix. As for kanji... well, that's a big hurdle, and you want to pace yourself, learning 5 or 10 a day. If you just drop into full kanji mode right away, I expect you'll be endless lost with similar-looking kanji. (Some people do have excellent visual memories though... I'm not one of them, I mixed up kanji endlessly until I went back to the start at systematically learned them with Heisig's mnemonic system, 'Remembering the Kanji'.)