Seeing as you have said central heating boiler rather than combi, it would suggest you need a system boiler.
I would definitely recommend a Worcester, especially if you get an installer who is able to offer a seven year guarantee (think you have to install the Worcester filter now to qualify)
I am a sparky and wire 2-3 boilers a week for a local firm and being honest i haven’t know a system boiler to give any trouble.
Admittedly they are basic but seem very reliable.
If you want a combi boiler, again i would definitely recommend a Worcester still, i can’t really compare to other brands but the customer service/guarantee has been second to none and that is the service i have received for my own boiler, not just customers.
Either way, i think your £800-900 budget is a bit optimistic for a quality installer.