montage said:
Got the feeling they will take the bike off me and leave me to limp home all lycra clad
They might. Recently I contacted Shimano about an issue I had with my disk brakes. They told me to take the bike back to the shop where they would remove the brakes and send them back to Shimano for inspection.
I said: "Not good enough. I could end up without my bike for weeks and I use it every day".
To their credit they then sent a full new set of brakes out to the shop, I went in and picked them up and fitted them myself at home.
I agree with Mr Paul's advice, it is right enough, but in this case I'd get what you want in place before you go out of the door. Phone the shop, tell them you want a new brake. Don't ask, tell them. Then you only go to the shop when they have the part so that it can be swapped over. It may be better to contact Shimano too, they seem pretty good to me and I'm sure they'll sort you out.