oh yes i should have said, when your choosing a boiler you need to think carefully about the power rating of the boiler i.e. the kilowatts or kw.
most central heating systems, providing its not a huge house, can be run off a relatively low powered boiler, lets say 24kw, the bulk of the power of a boiler is used for the hot water i.e. the higher the kw of the boiler the faster the delivery of hot water you will get. if you buy an underpowered boiler then you may find that you have to run a bath really slowly to get the water hot enough and then it takes so long to fill that the water is cold before you get into it. the downside is the higher the power output the more expensive it is to buy and run.
you need to get this sorted from the begining or you could end up unhappy. for an average 3 bed semi with perhaps 3 hot water outlets i would have thought a minimum of 28kw would be needed and preferably 30kw. you need to dscuss this with your installer.