I was going to DIY source my leak and recharge with the DIY kit from Halfords that repair minor holes in non metal components, but chickened out.
I was also going to take my car to an independent aircon place, but decided in the end to go main dealer because of the 40% off labour and 10% off parts benefit and pay a little extra for piece of mind with Tony Purslow.
It transpired my condenser was leaking (I was kinda suspecting it was a metal component) and the cost was £306 inc VAT for the parts and labour to fit with some new rubber seals/rings (the front bumper on the W210 also has to be removed which bumped up the labour element). Then £110 for the recharge and the original leak diagnostics and new UV dye just in case - total £416 inc VAT, not cheap, but just borderline for me but also reassuring the correct part would fit properly.
Anyway if you want third party condensors then it's much cheaper and this will give you an indication of the bottom end of the market PART ONLY! + delivery;
http://stores.ebay.co.uk/AutoCool-North-West_Mercedes-A-C-Parts_W0QQcolZ4QQdirZ1QQftidZ2QQtZkm
Mercedes C Class (W202 Series) A/C Condenser Radiator
£125.00
Mercedes C Class (W202 Series) A/C Condenser Radiator
£125.00
Mercedes C Class (W202 Series) A/C Condenser Radiator
£128.00
Mercedes E Class (W124 Series) A/C Condenser Radiator
£98.00
Mercedes E Class (W210 Series) A/C Condenser Radiator
£105.00
http://stores.ebay.co.uk/Aircon-Medic_Mercedes-Air-con-Parts_W0QQcolZ4QQdirZ1QQftidZ2QQtZkm
Mercedes (W126 SERIES) air-conditioning condenser
£110.00
Mercedes (W129 SERIES) air-conditioning condenser
£150.00
Mercedes S CLASS (W140 SERIES) air-con condenser
£130.00
Mercedes 190 (W201SERIES) air-con condenser
£165.00
Mercedes C CLASS (W202SERIES) air-con condenser
£145.00
Mercedes C CLASS (W203SERIES) air-con condenser
£180.00
Mercedes CLK & SLK (W208SERIES) air-con condenser
£130.00
Mercedes VITO(W638SERIES) air-con condenser
£115.00
Mercedes E CLASS (W210 SERIES) air-con condenser
£115.00
Mercedes S CLASS (W220 SERIES) air-con condenser
£135.00
Mercedes M CLASS (W163 SERIES) air-con condenser
£125.00
/EDIT
I've noticed these two merchants tend to concentrate on condensers rather than compressors, evaporators and receiver driers - so I guess condensers must be more failure prone.