hmm, that's interesting Julien - I can find no information regarding blight like infection of Kerria in the UK, but there certainly is in Canada, see link below for information. If its that, someone needs to know about, like the RHS for instance - it could be another unwelcome globalised infection...

http://www.torontomastergardeners.ca/askagardener/how-to-deal-with-blight-on-kerria-japonica/

Ours often loses stems for no apparent reason. In fact one of them I noticed today has gone completely. Possibly drought.

Thanks bamboo, just found pictures of the problem on a sight called home garden yard Illinois America the problem is called phomopsis canker have you heard of this and should the rhs be informed, here in derby it's devastated so many plants.

Well, its certainly worth contacting the RHS in case they want samples for ID of the problem. I've not heard of it here at all as an issue, but these days, and with the long list of diseases and infestations on the 'probable threat' list, who knows. It might be some other problem - do you have any infected plant material you could send them if they want it?

Thanks, yes I have plenty of material, will make contact with them and see what they say.