David Sims wrote:
Currently:

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

ecoTEC plus 637 - 12.0 - 37.0kW
40000-126000 Btu/h

This one isn't actually downsizing, it automatically varies the power
between 12 and 37Kw.

How often do you take two showers (as the household, presumably you
don't yourself) at once?

Is the property insulated? Walls, floor, ceiling?
About what is the external wall area?
As a rough guide, what's your heating bill over the year?

Do you want to downsize as the new one is cheaper?