New Boiler - noisy operation

Hi i recently fitted a new system boiler to my 'y' plan system.
The original boiler was rated at 56,000btu.
The new boiler was rated at 85,000btu (the lowest you can buy it seems)
When the system is running it seems to make a boiling sound like a kettle and the boiler goes on and off a lot - would the rating of 85,000btu be to high for my system and after looking in the installation instructions would the main burner pressure need to be reduced to lower the BTU to prevent it heating up the water in the system to quickly and cycling on and off?

How High is your boiler thermostat, If the boiler stat is too high and the boiler is too powerful for the system there is a chance that the Heat Exchanger could begin to kettle (boil).
Are the other components on the system very old, it wll be worth checking to see if the pump is working as it should, and make sure the valve is kicking over properly.

Two things.....

First, how many goes do we get at guessing what boiler you have - 10? - 20?

Second, what does the installer and commissioner of the boiler have to say about it? Or did you do it all yourself?

Alpha do a 45000, Potterton a 57000...
Assuming you did it, how did you clean the system before changing the boiler?

What does the CORGI registered installer say about it?

Tony