I found a couple of more places that seem to have more reasonable prices for this part
2028207310 [2028207310] - £37.00 : Airflow Ltd, Air flow meter | Idle air control valve | Blower regulator
BLOWER REGULATOR - Heater Blower Regulator Mercedes C E SLK Class RI6004
Don't know anything about these people but at those prices they might be worth a go. Probably find out soon as my fan will not turn all the way off