Thank you Jessica
I attended a few events last year with Slaughters and their winter workshop where we were told to make the cover letters concise. They want a brief intro about you, i.e. what you are studying/studied, where that is/was and at what stage you are at now. They want a second para on why commercial law and a third para explaining why slaughters. They say if you have mitigating circumstances then this should form a fourth para but otherwise just sign off. Make sure to sign off appropriately with 'Yours sincerely' as you will be addressing your letter to somebody (Janine Arnold is the recipient for this year still I believe). Make sure it looks professional and appropriately laid out - this matters as much as your content. Check you are using the correct date and all the address details for the London office. I also added a brief line before signing off to state that my CV was attached and to say that I looked forward to hearing from them.
I hope this helps and if you have any further questions please feel free to message/asl in this thread