Thanks for all the knowledgable posts in this thread. I'm a bit wiser, which is something.
Maybe I'll just go for the most recent CORGI award winner - Baxi DuoTec 28. Plus flushing, plus a Magnaclean, plus a corrosion inhibitor. Although I'm looking at ~£300 for flushing, which is rather pricey.
My current boiler is 24KW and has been adequate, so I'll go on the price difference between a 24KW and 28KW version of the same model.
I'm now wondering (a) how on earth my coworker got a new boiler supplied and fitted for £800 and (b) what wasn't done with it.
THeres things to consider, and one is that British Gas aren't always that expensive, local guys, in no way all but sometimes will be not doing some of the work required to be compliant with every little last regulation. Plus they'll be speccing things like a power flush, which costs signifticantly more than a simple flush, and things like hiring a fully spec'd electrician to do work if the boiler is in the kitchen, which it often is.
But British Gas are nearly always negotiable from what I've seen, get 3-4 quotes, they should be free, with an itemised list of costs, and compare what you're actually getting.
Keep in mind things like, if the system is old and likely has gunk built up in the system, when you file for a warranty replacement of a boiler, they'll come and check it and if they find the system all gunked up because it wasn't powerflushed, you'll find somewhere in the T&C's of the warranty contract that you aren't covered without a quality powerflush before fitting the new rad, as the easiest failure point of a boiler is gunked up heat exchangers which get utterly screwed by gunk in the system.
So that could be £1k for a powerflush listed by British gas, against a £300 drain/flush from the local guy.
British Gas are pretty good to get a quote from purely because they'll likely do everything you need to be compliant and forfill all warranty requirements so you aren't screwed in the future, and at worst you have a great list to work off when asking for work off other local guys/companies.
As others have said, Valliant are very highly rated, look around on plumber oriented forums who talk to each other and ask each other for advice on jobs they are doing. Then see which boilers they say break down constantly, and which rarely if ever break down.
You'll see Vallient, Baxi, Veismann and Worcester Bosch as the main ones to go for.
In terms of power required, again your several quotes and the guys who come to do them will have a very decent knowledge of what you need, which will be determined by, how many bathrooms/showers you have, how many are likely to be used at the same time. I forget if combi's are determined purely by their central heating power, or just by bathroom numbers, either way, again, quotes/gas men can help you out here and recommend the right thing.
Not sure if theres still vouchers available, but if you have a crappy rated one, which is slightly less likely at 12 years old, then you might be eligible for one of the £400 off vouchers, and n-power/british gas are matching the voucher so you can get £800 off a new install, which if the quote is competitive(considering what they actually want to do in terms of work), then that could be the way to go.
AS for your coworker getting one for £800, if its just a crap boiler but a very newly fitted system, well its perfectly possible for a boiler to just give in and be literally a drop in and replace, which will be the cost of the plumber for a few hours and the boiler and not much else.
On a 12 year old system, a basic flush would be required at the minimum, powerflush maybe, depends often on how well the system was originally put together.
But this is also why local guys or cowboys can be very cheap, not adhere to any of the regulations, ignore safety issues, drop in a new boiler and do very little other work, then you find yourself in 2 years with a broken boiler which the manufacturer won't replace due to the crappy work. At which point you have to pay for a new boiler and probably all the work done properly like you should have done the first time.
Thats why fully itemised quotes are important, so you can compare what you're actually getting.
Another reason British Gas are often expensive is because, well our plumber is very highly rated, not british gas and he has a whole host of other qualifications so he and his team can basically do anything you need building/plumbing/electrician wise. However he's doing a bathroom for us and isn't available till March. British Gas have so many people you can likely have it done within a couple weeks. But the numbers of people, coverage and availability mean by and large the company costs more to run and so they can cost more. But then a company that knows people need a new boiler and a fixed system, know if they can offer it done WAY before most plumbers, they can also charge more for that privilege.
. Though as mentioned I rated Vaillant higher (had so few problems with them compared to some others). I only fix them too, not install (once in a blue moon only). If you kept a spare water pressure sensor around, you'd probably never be without the boiler for more than a few hours while you wait for someone to come fit it 
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. I expect there are numerous install defects too (condensate route and termination being a common one). It won't last long before something goes wrong, I'd put money on it.
