In order to compete in the ever increasing pixel race, Canon increased the pixels of the 50D. Did they increase it to a point of diminishing returns? Some reviewers state "possibly yes"
http://www.dpreview.com/reviews/canoneos50d/page31.asp
"It appears that Canon has reached the limit of what is sensible, in terms of megapixels on an APS-C sensor. At a pixel density of 4.5 MP/cm² (40D: 3.1 MP/cm², 1Ds MkIII: 2.4 MP/cm²) the lens becomes the limiting factor. Even the sharpest primes at optimal apertures cannot (at least away from the center of the frame) satisfy the 15.1 megapixel sensors hunger for resolution. Considering the disadvantages that come with higher pixel densities such as diffraction issues, increased sensitivity towards camera shake, reduced dynamic range, reduced high ISO performance and the need to store, move and process larger amounts of data, one could be forgiven for coming to the conclusion that at this point the megapixel race should probably stop. One consequence of this is that the 50% increase in pixel count over the 40D results in only a marginal amount of extra detail.
We're by no means saying the 50Ds image quality is bad but it's simply not significantly better than the ten megapixel 40D. In some areas such as dynamic range and high ISO performance it's actually worse and that simply makes you wonder if the EOS 50D could have been an (even) better camera if its sensor had a slightly more moderate resolution."
I am not really cognizant regarding U.K. prices, but I think that you could likely buy a 40D and keep your present 400D as a back-up for the price you might pay for the 50D after selling your 400D.
A back-up camera is not just an investment; it is an insurance policy against your camera failing and thus losing out on any photography of an event. (Cameras fail in a direct relationship to the importance of the subject being photographed and in seemingly an inverse relationship to the availability of backup gear). It is also nice to work with two cameras, each wearing a different focal length lens.