In reply to Philip:
Trade-in will not equal Ebay price. Shops they have to make a profit on 2nd hand goods and pay VAT on the profit element on 2nd hand sales.
On Ebay: for lenses, I usually work on a rule of thumb of 75% of new price for 1st party lenses (Canon, Nikon) and 50% for third party.
Camera bodies tend to depreciate to a base line (even very old DSLR bodies still seem have some value on Ebay).
The 400D should go at c. £140 on Ebay, you'll be getting <£100 Trade in.
I would expect to get c. 50-60% of the Ebay price via tradein.
Harrisons in Sheffield do a brisk trade in 2nd hand - but be prepared to take a hit if selling to them!