[dirtydog]

Worcester Bosch are probably the market leaders for boilers purely for their well deserved reputation but Veissmann boilers are also very good.

As you've already got a combi boiler I'd stick with that otherwise you're looking at big bucks

I would start by testing the pressure of your incoming wate mains as that could dictate what boiler you go for.

Check boiler flow rates

Most of the manufacturers have boiler calculators on their websites

Also budget for the system to be washed through as the heat exchangers in modern boilers are much smaller so will block up real quick