Where employees receive a car allowance instead of a company car, under the smoke-free legislation must the employer ensure that they do not smoke in any car purchased with the allowance?

There is no requirement under the smoke-free legislation for a car primarily for the private use of the person who owns it to be smoke free.

From 1 October 2015, it is illegal to smoke in a car, or other vehicle, when someone under the age of 18 is present. It is the responsibility of the driver to prevent passengers from smoking. The driver and the person smoking can both be fined £50. It is not the duty of the employer to ensure that a car that is primarily for the employee's private use is smoke free when someone under 18 is present.