Instructions on said duvet were to only machine wash in 9kg drum or larger. Mine is 7kg, from memory.
Second option was wash at cleaners. There are no self-service places around here, so I phoned a dry cleaners - yes they had a large capacity washing machine, and it would cost £19 
So I opted to bin and replace - £11 from Amazon.
Should I just have washed it in the sink somehow?