Below is a copy of a message David Cameron is sending to party activists, members, and staff this evening.
Sam and I have been overwhelmed by all the letters, cards, emails and flowers we have received about Ivan. Sending an e-mail this week just gives us both a chance to say a big "thank you". It means a lot to know that others are thinking of us and him.
We always knew Ivan wouldn't live forever, but we didn't expect to lose him so young and so suddenly. He leaves a hole in our life so big that words can't describe it. Bed time, bath time, meal time - nothing will feel the same again.
We console ourselves knowing that he won't suffer anymore, that his end was quick, and that he is in a better place. But we all just miss him so desperately.
When we were first told the extent of Ivan's disability I thought that we would suffer having to care for him but at least he would benefit from our care. Now as I look back I see that it was all the other way round. It was only him that ever really suffered and it was us - Sam, me, Nancy and Elwen - who gained more than I ever believed possible from having and loving such a wonderfully special and beautiful boy.