skipping ultegra 10spd chain

I've just replaced cassette and chain. 

As previously I've put on a new ultegra 6700 10 speed 11-28  cassette and an ultegra 10 speed 6701 chain. Both bought from wiggle.   Running perfectly on my big ring but skipping under load on the small ring.  6700 SS rear derailleur. The skipping feels exactly the same as when there's a stiff link.  There isn't a stiff link.  The indexing is perfect.   Skipping in the mid gears.  The engraving on the chain links is on the outside, that is the chain's on the right way round.  What could be causing that?  

Grateful for any thoughts.

Richard

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