I'm wondering, as an undergraduate i'm supposed to apply for PhDs at the start of my final year, does that mean my results from previous years of my degree (which don't count to my final grade) will be important. Or will I be able to make up for my previous slackness in the 2 months before the deadlines?

When I applied for a job in my final year at Uni [couldn't face a PhD straight away, needed to recover from the horror of finals first! ;o)]the academic who supervised my final year project provided a reference, which included my predicted degree result. This was based on my results from the 2nd year and first half of the third year i.e. results that counted towards my final grade. Why not have a word with your personal tutor or dissertation supervisor? I expect they will be able to advise you on how things work at your Uni.

Your tutor will give you a reference, with a predicted grade based on their opinion of you, combined with your previous results. If they are not prepared to predict the required 2(i), then you'll just have to find someone who will, or delay applying untill after your finals.