Boiler and Radiator Sizing

[hulahoopian]

I am having my bungalow's gas boiler replaced and am considering replacing my radiators as well. The radiators are probably original 1975 vintage, steel panels with no fins, mostly singles but some doubles as well. According to http://www.pipecasings.co.uk/downloads/download files/Radiator Output Calculator.pdf, they total about 18kW.

My boiler has some info written on it - depending on gas rate it's either 28.9kW or 31.3kW input and 20.5kW or 23.5kW output.

So here are the questions:

Given that since this system was installed (and presumably sized correctly) I've had 3 times more roof insulation added, plus cavity walls filled, do I really need a 30kW boiler as everyone seems to be trying to sell me ? Does it matter if it's "too big".

Secondly, if I do replace the rads, should I go smaller to get the same or less wattage than before, or just get something the same size so it fits existing pipework better (but would be higher output, since it will have fins now) ? Can the radiator outputs be "too big" ?

 

[PrenticeBoyofDerry]

To answer your questions correctly, heat loss calculations would be required.
This would mean measuring the size of rooms, finding the U values of the building fabric enclosing each room and any temperature differences between rooms, internal walls and external walls, plus heat loss through windows doors and air changes.
Then your rads can be sized and an appropriate sized boiler chosen.