I'm applying for a job I really, really want and was wondering if it's bad practise to mention a connection to the company in my covering letter? My boyfriend used to work for the company that I'm applying for the position with and they loved him and though very highly of him etc, so I don't know if it's worth mentioning that I've heard lots of good things about the company because of that? I know it doesn't sound especially professional but I'd love this job so much and they do say it's 'not what you know, it's who you know' and it might help give me an edge over other candidates. Should I mention anything to do with it or not?
Nope. You don't have a connection with the company - your boyfriend does. Don't mention it.
Not unless it has a recommended by field on the job application. If you know lots about the company that's great, they would think you've worked really hard into researching the company.