consider the W-Bosch. They have a good reliabity record and very good backup support. The Viessmann with weather compensation has got a history of reliability issues.
Look at:
http://www.vaillant.co.uk
Look at a the German Ecotec Plus 837. Its £1003 inc VAT, that is very well priced for a 37kW boiler. Cheaper elsewhere.
http://www.vhsdirect.co.uk/site/department.cfm?id=A2BB0430-A55E-C4A8-97A5CE4EAF584832
Outside sensors are extra!
Have a Magnaclean on the return to the boiler. That will prevent any blocking up of the tubes in the heat exchanger. This applies to any boiler.
The Vaillant can come with a 3 year guarantee, or even 5yrs from some installers. They use specifically designed parts by Vaillant in brass, meaning excellent quality. The Vaillants are better quality than Broag and competitively priced for a superior boiler. They are well priced quality boilers having a superb control system with integrated outside weather compensation. Buy the outside weather temperature sensor and use weather compensation - great comfort conditions and economy. You will not look back.
The best price/performance around.