When you say you are going to install, do you mean D.I.Y? If so, then you can't. The regulations require that you are competent (meaning you have proven your competence and are a member of an organisation such as corgi).
Even just fitting an unvented cylinder require's a seperate certification.
If you are planning to have the work carried out by registered installer's then they will advise you.
Best plan is to have people in your area recommend installers they have used and get quote's from at least three of them
U:plumb
August 2007