Hi Paddy.
That 400 L lens is such a great lens of bird photography-you will not be disappointed-well worth the money,The Sigma lens you mention,is really a 'make do' lens-you will always feel a bit short changed in the field with it-therefore its not cheap-its expensive-as you will get home open up images and go 'H,mmm' open up the images from the 400 L-and you will go 'Oh yes!
)
I have used the 400 L on my 450D similar to your 500D,and have been very impressed with the results-right from day one-in fact I feel the results are equal or better than results from my 50D.
I have changed my approach to using the settings to Dennis,lately-I have started using narrower AV-very important when close to small birds.Partial metering instead of spot,and AV mode-dial up required exp comp as needed,or lower/raise ISO
Good luck with your purchase whatever it is.Here is an image I took of Variegated Wren last week,iso320,spedd 640,AV 8,partial metering,with 50D