Illustrator charge rates

Hi I know this isn't an easy question to answer but I do need some advice here. I am working for a creative agency making artworks for vinyl applications in leading high street stores. At the moment to artwork designs (which are part stock vectors and party artwork myself) I am charging £30 per hour, the agency doesn't have this info as I charge an entire fee for various art-working of visuals for their client etc in photoshop etc. Can anyone give me a ballpark on what would be a reasonable hourly rate for this kind of work and if the agency decided to employ someone to do Illustrator/photoshop work what rate this might be. I started this kind of work when I didn't know illustrator that well and its been a year now and I have become more experienced which is why initially I charged £30. I know for some this is a sensitive subject but would appreciate any direction as I dont have firm contacts in the industry to ask, thanks if you can help, Sam