Main dealer for a spare/second key is the better option, even though there maybe a couple of slightly cheaper alternatives mentioned in the forums here, but for peace of mind a genuine MB key is what I went for.
DO NOT buy from ebay thinking you can get one reprogrammed.
Are you sitting down?....................
£227 all inclusive of VAT and includes new metal blade key. You will need the V5, proof of identity (passport/driving licence) and a recent utility bill. Takes about 5 working days to arrive pre-programmed at the dealers.