go check out rc groups and look up the blade runner or the wild wheels heli, they are the exact same thing. Sounds like your problem can be comming from two things, either one side or the other has too much or too little pitch that causes one side to create more or less lift than the other side resulting in the vibration, or the blades are not the same weight, which would also cause the vibrations.

So basically I would see if there is anything you can do about the balance or checking out if the pitches look the same, and it's producing the same amount of lift on each side. Also, I dunno about the UK, but here in the US they do sell replacement tail rotors for that heli, and you could just get another one and see if that fixes your problem.

To balance the rotor, balance it on a metal ruler or a knife or something with a well defined thin hard edge. If it balances right in the center of the hole, then you've got it. If it is out of balance you could try very small peices of tape, or filing part of the blade. If you want more info PM me with your e-mail and I'll send you a balancing guide for a little bit larger heli's, but it could be scaled down to this size quite easily.

It could also be a deal with harmonics, but that is quite complicated, and I'm not sure there is too much you could do about that.