So for flues if you are unsure measure the flue you have, just because its there does not mean its right. Incorrect flues will restrict the performance of the unit. Your installing Gas Safe engineer will do this for you. 

You cant have a smaller size flue for a bigger unit so if you have a 100mm unit you cant fit  90mm flue. You can go the other way thought. So you could have a 100mm water heater and a 110mm flue. With an adaptor.