The 42CDi has been suggested because you have two bathrooms. The selector assumes that you want both baths filling at the same time and that you have sufficient flow to enable this to happen.
Presumably you have chosen a combi as you want to get rid of the water heaters and don't want to install a hot water cylinder. Whether this is feasible depends on the incoming cold water flow rate and pressure.
If you want two baths filling at the same time
from a combi boiler you need a flow rate of at least 20 litres/min for a 35C temperature rise. Neither boiler suggested would be able to meet this requirement.
You can easily check you flow rate at the cold tap in the kitchen using a litre-marked bucket and a watch. If you have a garden tap straight off the incoming mains, check the flow with garden tap open and then with it closed. The lower flow rate is the significant one.
The old boiler is 15kW, so your heating requirement is about this, but may be lower if the insulation has been improved since the boiler was installed. You can check what you now need by using the
Whole House Boiler Size Calculator.
You should also check that your radiators will be suitable as they will give off less heat when fed from a condensing boiler. This is because a condensing boiler runs at lower temperatures. Us the information in the
Stelrad Elite Catalogue to find the outputs of you rads (assuming they are modern steel ones). You should use the page headed 40Δt as this takes into account the lower outputs when fed from a condensing boiler.