I've opened the bonnet to discover that the oil cap is off, and oil is all over everything. How can I get it cleaned up, and what damage will it have caused, if any?
It shouldn't have caused any damage, as long as there is still plenty of oil left in the engine.
To remove it you will need to use some kind of degreaser - you can buy proper engine degreaser at many automotive suppliers. It is probably worth getting the engine bay properly steam cleaned however, as you'll never be able to get into all the little holes and corners yourself!