My Camellia tree seems to be dying - help!?

Hi, I live in London, and have a beautiful pink Camellia tree in a small sheltered north-facing garden in a raised bed. I have lived in the flat ten years, and it was well established when I arrived. It is about 2m high. Since I moved in, it has flowered every year, and grown and looked very healthy. In 2008, I bought a bag of camellia fertiliser and dug it in the soil. Over this winter (2009/10) it produced more buds than I have ever seen before. I was expecting a huge spring of flowers. However, in the last couple of months, the buds have been falling off unopened. leaves are turning brown and falling off. All I could think of was that we had a long and cold winter in the UK this year. I have read some advice online, and checked for spiders under the leaves, and found none. In the last week I have also loosened up the soil around it in case of root rot. I am thinking of now digging a trench next to it to increase drainage, and maybe digging a lot of soil from around it and replacing it with camellia fertilizer. Having said that, the tree thrived for a decade without any intervention, so I cant see how it can be a soil problem. I would appreciate any advice you have. It is such a beautiful tree, and I want to do anything I can to save it. thanks. Chris