I am just about to take delivery of a C180K Avantgarde SE estate; I managed an 8.9% discount with a Midlands MB dealer.
Be aware that the facelifted C-Class is being built from this month for the LHD markets; available in the UK from next Feb from the South African RHD saloon factory. However, the dealer will play this down but you should use this as leverage for a 10% discount .
Expect by year end for M-B to offer special deals on the C-Class (free leather, Xenons, etc) as they try to mop up unsold new cars before the facelifted model appears. M-B London have over 47 new C-Class's in group stock; try for a 10% discount.