I am in the same situation. I have not registered my business yet and after completing my first project last week, now have a cheque written out to my company name. I have just opened a business account and just waiting to receive the account details etc. Today i completed as form to informed HM Revenue and Customs of my business. As long as you notify them within three months of you starting up it's ok. There is a helpline number you can ring for advice: 0845 9154515. If you open a business account asap in your business name you should get your account details etc within 2-4 days and then you can cash your cheque.
Hope this helps and makes sense, sorry if i've waffled, not joined a forum before!
