Have a look at Archie Kidd boilers, simple, old-fashioned, suitable for
open systems, high efficiency and build quality.

So old fashioned they don't do "that internet thing", and not much web
info via other companies:

http://www.hwch.co.uk/kidd.html

Very good postal literature (lots of general guides and independent
revues) and free on-site surveys.

Archie Kidd (Thermal) Ltd,
Poulshot,
Devizes,
Wiltshire,
SN10 1RT

Phone: 01380 828123