Things will settled down, dont have too high expectations of your self at this stage, you are there to learn because, did you not know you only learn how to nurse after you qualify. I presume as you are in the UK that you will have a preceptor and that you will be under that person for 6 months. I too worked in Acute renal after I had qualified many moons ago but not that much has changed, it can frighten you because the patients can be really poorly but remember they are not your ultimate responsibility as a D grade you will always have a senior more experienced nurse above you. Ask questions all the time, dont undertake to do anything you are unsure of ask for help. Dialysis looks very technical but again you will learn quickly how to do it, you dont need to be qualified to set up a haemo machine or do CAPD you just need to be trained. You will learn in time how to needle fistula's how to look after tenchoff's. Just learn now how to nurse, time manage and familiarise yourself with equipment, term's and phrases, settle into your role you will get there.
As for not being prepared properly to be a qualified nurse by the RN's, if you trained for 10 years you would never be prepared. You will never see everything nor do everything in your life, i am working with a nurse qualified for 32yrs and has never ever seen an arrest!
I ma sure things will work out well for you enjoy this time of being supernumery-because soon you will wish you were again lol