I recently bought a Ford C-Max 05, and have found that when I am stopped in traffic a strong smell of petrol or oil comes through the air vents and there is white smoke coming from the bonnet. Does anyone have any suggestions on what this could be?
Smoke under the hood can be caused by many things. It is commonly caused by oil leaking into the exhaust manifold(s) from the valve covers where the gaskets have gone bad. Another common reason is sloppy adding of oil that missed the filler. You will have to identify it to know for sure.
As far as the smell coming from the vents, on most cars the fresh air intake for the cabin is located in close proximity or the engine compartment. Therefore, when the smoke from the burning oil is present, it can be pulled into the fresh air for the cabin, thus allowing the smell to come thru the vents.
You will need to inspect the engine for leaks. Once you have found them, please update your question so we can help with more detail. Hopefully it is just valve cover gaskets, which can be a simple, inexpensive fix.