Is there a financial benefit for PhD student to join a funded research project
I'm not in the U.K., but in my part of the world, it's common (one might say usual) for computer science PhD students to be paid. Is the determination of whether a PhD student is paid related to whether the project is funded? Not necessarily.

When is it appropriate to ask about payment?

Should you talk to your supervisor about this? Instead of asking 


  Is it normal to not be paid?


ask yourself


  Is it unusual to be paid?


In any scenario in which is is not unusual to be paid, it is neither inappropriate nor offensive to (politely) ask whether you can be paid for work you are doing.* In your case, it is definitely appropriate. So, go ahead and talk to your supervisor about this.


* Here is an example of a scenario where it is not usual to be paid, and it was therefore inappropriate to ask for payment. As you can see, it's an extreme example :)