Rear sprockets have a load of play.

[cosybike]

Bearings collapsed?

The chain jumped off at a gear change. When I put it back on it discovered the rear sprocket assembly is wobbly and there may be some resistance in wheel turning.

It's done 1000 miles since January.

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[cosybike]

The wheel is still secure though.

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[brianw]

The wheel is still secure though.

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Have you had the wheel off and checked the locking nut that holds the cassette in place on the freewheel splines?
Obviously you need the splined extractor nut to do this.
I've been messing about with my rear cassette to reduce this chain coming off problem, if you are interested I'll post what I've done

 

[Fordulike]

As brianw mentions, it's well worth adding a cassette tool, plus chain whip, to your toolbox, for times when you need to replace the cassette, or check its tightness.

Deluxe Freewheel Bike Chain Whip Cycle Bicycle Cassette Cog Removal Remover Tool

 

[cosybike]

A nice bike shop in Hammersmith tightened it all up. Symptomatic of lack of factory and online shopping pre delivery inspection. The brake discs were loose as well when it arrived at first! (Although I've never had issues from two bikes bought in the past this way.)

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