Can one lookup the chassis number online somewhere if knows reg etc?

In most countries, it's not directly available online for privacy and security reasons. Instead, you may be able to obtain it from the government agency responsible for vehicle registration or from the vehicle's manufacturer/dealer by providing relevant documents and justification for the request.

In the UK, you can use the official DVLA website to look up vehicle information by providing the vehicle's registration mark (number plate) and payment of a small fee. This will give you basic details about the vehicle, including the make, model, color, and registered owner. However, the chassis number is not included in this information.