Just to add my two old pennies worth, I do install boilers of many shapes sizes and types, and without wishing to muddy the waters any more than have been already, lets get one thing straight about Combination Boilers.
They provide Heating OR Hot-water. NOT both at the same time.....
By this I mean that if your household tends to run one long shower straight after another (when the family descends on you at Xmas?), or indeed a bath, then another, then another, your radiators will cool off. Whether you notice a dip in inside air temperatures is down to the individual.
Also, do pay attention to the Manufacturers specifications about hot-water output rates, some of them 'inflate' their flowrates by using a 30degree temp rise, this week, your incoming cold-water temperature is unlikely to be above 5degrees, so even a quoted rate of 15litres/min at 35degrees (say) would not be adequate for bathing/showering or doing the washing-up!
A decent bath will take up to 30minutes to run Dec-Feb.
I fit a lot of combination boilers, and advise my customers of this BEFORE I get the pricing guides out.
My favourite? Viessmann 100 series. Not at the bargain basement end of boilers but, you get what you pay for.
Also, as regards powerflushing, I for one would not consider installing a combi without powerflushing the system properly, installing a magnetic filter, and using only Fernox/Sentinel inhibitors to the water system at the end of the job.
As to whom to get to do the job, recommendations only from people whose opinion you trust, and if you were to invite me in warmly and offer a cup of tea while I have my tape measure out and discussing YOUR needs, I may even accept YOU as customer!
Just my take on things.
Best of British
DH