Fabia engine management light.

I have an 02 Fabia 2 litre and ever since I bought the car secondhand from a Skoda dealer 18 months ago I have had constant problems with the engine management light coming on. Both Lambda sensors have been replaced and my dealer constantly turns it off, only for it to reappear soon after. He also told me I needed the carbon cannister filter changed as the code was showing a build up of petrol fumes, so I did. But it persists. There seems to be no definitive answer. I took it to a VW specialist who cleaned something or other under the bonnet and it stayed off for nearly 3 months. Is there anywhere in the London area that could get to the bottom of the problem as I am loosing faith in my dealer. Many thanks for any help.