The dealer should indeed be able to tell you the code for your stereo. I know I have complained about Wilsons in Epsom, but they not only told me the code for mine, but actually set it all for me, and for FREE!
My car is also an import, which can make it hard for a Renault dealer to trace certain bits. I had all sorts of trouble just getting a gaiter for my gear lever, the chasis number and Reg number together didn't show up on their computers, so they had to take down 'all' the numbers on the sticker inside the drivers door and finally managed to locate my car on Renaults central computer.
I do not see why they want to charge you £10 just to give you the code, it really shouldn't take them that much time if they know what they are doing, but if you really need it, £10 really isn't that much to pay. I'd personally pay the money to get it sorted.
Good luck!