We have an oil Aga range cooker. If we installed a Thornhill Range 3-oven and a separate boiler, surely it would use more oil than a 2-oven with a condensing boiler built in?

No, definitely not.

Cooker / boilers are two separate units in one casing, so moving the boiler to a different location will not increase the fuel consumption. This is a common mistake. People think, as with a coal fired Rayburn, there is only one power source / combustion chamber. With cooker / boilers there is no connection between the two units, and there are two oil supplies and two burners inside the casing. If you are cooking you do not heat any water, and if the boiler is on the cooker does not even get warm.

It is just a convenient place to put the boiler! Or is it? It’s no cheaper, you lose an oven, and the plumbing might not be anywhere near where the cooker is going.

You can also get external boilers now, which keep the noise of the boiler outside.

But probably the most common reason people have a separate boiler is that they have a chimney and want the cooker under it. The condensing oil boiler is balanced flue / low level discharge only, i.e. the flue comes out of the back so it has to go Against an outside wall. However, the cooking only model can be a balanced, or a conventional flue into a chimney.

Of course if you want a larger boiler than 21kW you will have to have a separate unit. There just isn’t enough room inside for a bigger boiler and enough space for servicing.