So many questions and comments!
OK, according to MB a system should be checked at least once a year. If required, it should be regassed.
The cost was approx £65 plus vat, so about £75 at a dealer?
Jay, I agree. Perhaps thats why in nearly six years mine never did need topping up. I wasn't a member of the "leave it on all the time" school of thought but will leave it on now.
Finally, it seems on the W210 model that if the refridgerant / lubricant reaches a critical level, it shuts the system down to avoid damage. I know this because when the EC light was switched off, the compressor didn't fire up. It does now.
Greg