The problem, Hetty that for jobs like roofing (or landscaping) you need to have a range of quotes, because many an elderly person has been persuaded to accept and pay many times the price of a job, because they have gone with the first person who quoted.
We are in the fortunate position where, over 25 years we have got to know tradesmen, whom we trust and like, so are able to use them without looking for other quotes, but with our landscaping work, for example, it is a major project and is going to be very expensive and we do not know anyone who can do it. So if we are going with an unknown person we want two quotes.
Will we just go with cheapest? no in a case like this we will be going through them with a toothcomb, because, inevitably they will suggest different ways of doing the work and we need to make sure that every contingency is covered. Price is not the only reason a tradesman doesn't get a job, lead time comes into it. One company, who were not asked to quote said they wouldn't be able to do the job until November.