^yeah, i know, a lot of people love the ZR. Sounds like you just got a bad one.
On UG, i put a list of all the good double-locking trems in a sticky for the n00bs to help them, and I included the ZR.
If i were you I'd contact your shop and see what they say. Don't be defensive, or aggressive, just state what's wrong, that it's out of warranty (isn't it?) and see what they say. If they're nice, and offer to fix it, problem solved.
if you've got a dodgy one (i.e. bad from the start) you're entitled to a new one whether or not it's out of the guarantee, afaik (check with the office of fair trading though, i'm not 100% on this)- something along the lines of "goods not fit for purpose". It'd be useful to get this info before going near the shop, so that you can argue (if you have to) with the confidence of knowing you're right.
I always start out super-polite, so that it's THEM forcing me to argue if they get nasty. try to make them look like the bad guys, lol- at least then if and when you do have to argue, you can do it with a clear conscience, lol.