Thank you very much for this information Meg. I have Cyclamen Hederifolium in my garden and it has clumped up - it spreads very easily. It has been there for some years and the dogs have not touched it but I will ask the gardener to remove it now that I know about the risks.
Last year one of them suddenly started to munch on my Fargesia 'Simba' (a bamboo). It is a lovely, non invasive plant that I had had for some years but the gardener at the time gave it a very hard hair cut which resulted in new growth and it was this that the dog found attractive. Unfortunately it made her sick, although I was told it is not toxic. I decided to have it dug up but was sad to see it go.