Large 4 bed house 2 baths, 2 showers

Tanked hot water with Y plan heating (and now a thermostat !)

Ideal Mexico RS125 65% efficiency 37KW
depending on burner setting which is either 15.5 or 10.5 mbar 1000000
or 125000 Btu/h

If we replace it with a Vaillant can we downsize

The 30kW will be fine provided you don't have a leaky listed property.

If it isn't fine, you should spend any additional money on insulation rather
than a bigger boiler anyway.

Christian.