E-cigarettes are not covered by the ban on smoking in enclosed workplaces and public places. However, an employer does have control over whether their employees can smoke them while at work.
Good practice is for electronic cigarettes to be subject to the same general restrictions in the workplace as tobacco so they should not be used in any indoor place. This is not just because the risk to others is unknown, but also because it can be confusing if people are seen to be “smoking” what can look like a cigarette. This undermines the smoking ban.
It’s a good idea for employers to designate separate areas for users of electronic cigarettes and smokers of traditional cigarettes. If a worker is using electronic cigarettes to help them give up smoking tobacco then it is not going to help if they go outside to the same area as smokers.