Little cog slipping.

[onlyhuman]

Only the smallest one of the six. It slips under load. Is it knackered?

 

[spandex]

If it is slipping you need a new chain, freewheel and maybe a new chain rings.

 

[onlyhuman]

Not that kind of slipping

Sorry lads: I don't mean the chain was slipping, it is the cog itself. If I hold the back wheel and push the pedal hard, the cranks go round, the smallest cog goes round, stiffly, and the other cogs stay still.

 

[peanut]

sounds like your cassette lock ring has come undone. The smallest cog only locates on the freehub spline a little less than half its thickness. I'd suggest you remove the wheel and loosen the lockring. Check the smallest cog splines are not stripped or burred and retighten.

 

[onlyhuman]

Thanks for the suggestions, the block was in fact knackered and a new one is in place.