Where can I buy cheap kero?

Is it still dyed? Don't boatyards only have a single pump and just apply duty to filling the engine tank? Or proportion it to heating and engine, when there is only one tank.

Even with two and with red dye, it would hardly be difficult to have a hidden valve to switch to running the engine off the heating tank, which appears to only be connected to the heating.

Correct. Some boatyards specify the proportion, others ask the customer. It's all recorded. Fuel is also allowed for electrical generation with the main engine. Most splits are 25/75% propulsion/heating etc, or 50/50.

I've never seen an inland boat with two tanks. No need for sea going vessels.

Even 100% off road isn’t good enough to be sure you can use red, a near neighbour runs a wood yard behind the aforementioned garage and is within

GH

The only differences between red, and white diesel, is that red has a dye included to identify it, and you pay more tax - both work absolutely the same in an engine, or a burner.

No, heating oil is slightly 'heavier' (I think it's that way round) than red (or not) diesel for engines. Hence the (small) change required in the jetting of a boiler to use vehicle/red diesel.

I thought there were slightly more additives in road diesel, but yes, broadly the same. Kerosene is slightly different as is Jet-A and friends (JP-8)

I remember always thinking how airports smelt slightly of paraffin, and the old diesel tractors of my youth smelt way different to the diesels of today.

All of these products are fine-tuned to their intended use. After a horrendous experience last year with my AGA sooting up, I now buy premium kerosene home heating oil. The oil boiler was fine on the sooty muck, but the AGA was not.