Chain or Cluster ?
 

My chain is skipping on the lowest gear on my rear cluster,I`ve just replaced the chain[which I thought was in time]It shifted better than new for the first 200 km`s and now this skipping thing.Without stating the obvious[new cluster]has anyone got any different idea`s,the teeth don`t appear to be worn.

If the cog isn't obviously worn, it may just be the derailleur adjustment. Perhaps the stop is set a little too close.

Check the L-stop on the deraileur. Dirt is right you may have it set so it can come in too far.

First, let me ask this. The gear where the skipping is worse, is that the gear you use most of the time?

If so, then yes, your cassette is worn out as well.

What happens, is that as a chain wears, the overall length increases (bigger and bigger gaps at the pins, rollers, and links), well, as the chain lengthens, the teeth on the cassette wear equally.

If the new chain shifts well in the gears that are not worn as much, but skips in the gears that were ridden most of the time, you can bet you waited too long to replace your chain and now have to replace the cassette as well.

I'd suggest also buying one of these:http://www.parktool.com/tools/CC_2.shtml