Oil and filter changes are not covered by warantees. They are part of the normal maintenance of a vehicle.
Some condensation inside the engine is not a serious problem. It will soon get burnt away with a little use.
What could be a problem is if there is leakage between the coolant and the oil. You would be more likely to notice this by seeing oily deposits in the coolant rather than the other way round. If this is the case, you may find that the car overheats with the problem getting worse as time goes by until you need to replace the cylinder head gasket (a full afternoon's labour plus maybe £50 parts). If there are oily deposits in the coolant, you should write to say that you are concerned about the state of the cylinder head gasket so that, if things get worse, your letter will provide evidence that the problem existed while still under guarantee. Then you should drive the car as normal but keep an eye on the temperature gauge and fluid levels (you should do this anyway).
My guess is that there is nothing to be concerned about and the deposit came either from a bit of condensation or failing to have regular oil & filter changes or using a poor quality oil. An oil and filter change is certainly worthwhile. It will cost you maybe £20 plus the cost of the oil and the filter if you avoid the main dealer. If you use the main dealer, you are going to get stung with a big bill.
If you are concerned, you should write the above letter, be happy that the car drives well at the moment and hope that it continues to do so.