My CLK270 CDi starts off the key but then stops if I keep the key turned it will start after about 25 seconds I changed the glow plugs but that hasn't helped?

It sounds like your CLK270 CDi may have an issue with the fuel system, possibly related to the fuel pump or fuel filter, which could be causing a delay in fuel delivery to the engine. Additionally, there could be an issue with the engine management system or sensors, such as the crankshaft position sensor, affecting the starting process. Consider checking for any fault codes with an OBD-II scanner to pinpoint the problem more accurately. If the issue persists, consulting a qualified mechanic is advisable for a thorough diagnosis.