also will they see me as a financial risk due to only having a years worth of books?

Yes, they will. I haven't applied for a mortgage yet but last time someone at my bank (Co-Op) asked if I wanted one was since the crunch. He said they accept two years of accounts (I'd heard three). I assume the more you have the better.

Have you filed your first tax return yet? If you haven't, it might help to get an accountant to do it as apparently banks believe what they say you earned more than they'll believe you. You won't get much into it it if you only go up to last April though, but you can choose to submit your accounts for a period that isn't the same as the tax year, so maybe you could submit accounts for your actual first year of working, I'm not sure. If getting an accountant is an option you could ask to talk to someone at your bank to find out if they think it's worth it.

You're not applying on your own, so that makes a difference. Your boyfriend's salary is going to help you, and if you also have a good deposit that'll really help. Try this mortgage finder, where you can input all the important details about you and your partner and see which you're likely to be offered.

Your lucky its joint, you might get away with only having a years returns unless you are trying to borrow the maximum amount on your joint income.  Your partner will obviously been seen as the more secure half of the partnership.