Sole Trader/graphic designer and HMRC

Hi,
I don't know if somebody already had this problem, but if yes, then would be nice if you could reply 🙂

I've been always wondering (couldn't find answer on internet). What if... :
Example:
Client wanted let's say: web design and paid 50% upfront e.g on December, but project was finished January and balance was paid by client MAY 2013 (which means after tax year 5th April)
How to deal with this? When it comes to Tax Return.

:eek2:ohwell