i/ we instal 2-3 boilers a week.. worcester bosch all the way.. they can cost more to buy but in 6 years i ve only had one warranty call and wb sorted it within 30mins next day.
repairs, you find that a lot of wb parts are so much cheaper that for many other manufacturers that where possible for generic parts we fit the wb original branded product..
boiler 'sizing ' by the number of rads today is balony.. for a combi be steered how much hot water you can possibly demand at the same time.. ie how many hot taps have you.. the number of rads is largely irrelevant as WB even supply an akternative 'chip' for every combi to reduce the gas useage during the CH cphase.
the most common sizes we fit are the 24i jnr and the 30si.. only difference is the volume of hot water delivered and on the si you can control the hot water temperature..
cheapest place to buy the boiler is online at plumbnation.. then get someone to fit for a daily rate plus materials.
using this basis you can have a combi replaced with a 30si installed legally for under 1300 quid..