I had a problem with the remote control's on my 97 primera. The remotes kept losing the coding - then after a while when you locked the doors they would re-open straight away. I just got rid of remotes and used key to lock & open doors.
There was an article about this in the Sunday Times yeserday-it loosk like the older the key, the more it costs to replace.
I will have a read and see if there are any workarounds.
Was there not two sets with the car? Myself and my gf had the same problems when we bought secondhand cars recently. The sets we got with the cars were on their last legs, but the second sets were perfect.
My son bought a Nissan Micra 00 reg.(his first car). When he went to collect the car he was only given one key. I immediately got on the case and requested a second key and after much huffing and puffing one was provided. It was basically a sharp salesman taking advantage of a young 20 year old and "fobbing" him off with one key. Go back and kick ass....