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I would fit a vaillant ecotec 837. I have run two ex power shower of it. The vaillant is in my opinion far better quality. It has a far better pedigree than any of the worcester range. The only reason to go with the worcester is if it has to be floor standing. Worcester are not the same manufacturer they used to be and are still living on their old reputation.They use too much plastic. I have been called to no end of greenstars with pinholed plastic. You will find the installers with the best reputations in your area fit vaillant. If you must fit a worcester the 440 is there best offering.

The first quote of £4900 is taking the p*** Grit bas by any chance.
 I also would insist on a magnetic filter they make a lot of difference 
to the efficiency and longevity of the system. 
I still think you are better with a vaillant and you'll get a better warranty. Vaillant give our customers 3 years. If you buy the same boiler off who I suspect you got your first quote you get 1.