Don't be put off the forum by your first subject, stingow. It was unwise to use the words "ripped off" and some of the plumbers and heating engineers here are very sensitive and can appear quite hostile which is contrary to the ethos of the forum. Some of them, however, are extremely knowledgeable and if asked nicely, are kind enough to disseminate their knowledge in an altruistic manner. A few, I'm afraid, are boors.
At the risk of upsetting the pros here and as a consumer in this regard myself, the customer doesn't usually want the cheapest price, they want a professional job, ideally some sort of forward guarantee and good value.
If the customer doesn't understand the amount of work/experience/skill/equipment/extra parts etc. required for the job, he is left looking only at a final figure and the one thing he can cost himself which is the boiler and wondering if the difference is fair, which is exactly the reason for his post.
There is no evidence that stingow is a plumbing "tyre kicker" He needs a job done, has money to pay for it but needs the knowledge to understand what he might need and that what has been asked is reasonable. The gentleman providing the first quote has evidently not been able to explain what makes up his quote in a manner providing this knowledge.
If there were no further parts required and it took one man a day to fit it, would be unduly expensive whereas if it took a week for two men to fit and a thousand pounds worth of parts it would be unduly (and thus worringly) cheap.
In the absence of a recommendation, it is not unreasonable to have two or three estimates or quotations. Many businesses operate in such an environment
Offering a service to the public and then failing to reply or turn up to an agreed appointment is simply rude. There is usually only one chance to say "no" to a customer and whilst you might decline the work, you would still be asked again for a job you might be minded to quote for in the future.
When asking for a quote, I act upon the words by John Ruskin (1819-1900) who wrote:
"it's unwise to pay too much but it's worse to pay too little. When you pay too much, you lose a little money. When you pay too little, you sometimes lose everything because the thing you bought was incapable of doing the thing it was bought to do. The common law of business balance prohibits paying a little and getting a lot. It can't be done. If you deal with the lowest bidder, it is well to add something for the risk you run. And if you do that, you will have enough to pay for something better"