I'm actually quite embarrased now. I found it. I had to do an advanced search on the Waterstones site but it's there.
Doing a standard search on there for "Shades of grey jasper fforde" and only the pbook version shows up. I went through quite a few pages of search results looking for it too.
Of course as you'd expect the pbook is half the price of the ebook.
Appologies to Hodder & Stoughton and the finger of shame to Waterstones!