MOT question

What would you be thinking of fuel-wise?

I assume we're talking about diesel?

Aren't diesels only tested for smoke anyway? I imagine poorer quality fuel will be more smokey, so that might be something to consider. Even supermarket fuels made my old remapped mk4 Golf smoke more.

I'm not sure whether the emissions would be different or not, but regardless of that, I'd be more worried about the quality of it.
I work for a construction machinery dealer and we get diesel delivered into our tanks for filling the machines, and even though the diesel is white (a lot of customers can only run on white) none of our engineers will touch the stuff for their new vans. Even Tier4 machines are a concern when it comes to quality.
You'd need to consider sulphur and biofuel content.

Still assuming it's diesel, if it's ever had red diesel in it in the past then the filters will be stained red. The MOT won't check this, but if you live in a very agricultural area and you are suspected then you could get checked by another agency.
Red diesel has very high sulphur and will knacker a modern diesel engine, so any fines would be the least of your worries. A3's need ultra low suplhur diesel.

If we're not talking about diesel (red or otherwise) then forget everything I've said above

No they don't test for red diesel... although it may or may not pass emissions

so ypu just told the forum your useing red ?

Nope won't get picked up or noticed.

Red is exactly the same as white apart from the dye

We burn 1 litre of the stuff every 8 seconds in our brand new MTU marine engines. If it wasn't the same quality we wouldn't be able to use it due to warrantee issues

Red isn't the same as white, but with dyes in it anymore. It used to be, but not now.
Red isn't available as a proper ultra low sulphur diesel, which is why our new machines can't run on it. JCB, Isuzu and Cummins all issued warnings not to use red diesel in their Tier4 engines as it will cause damage and void any warranty. Any warranty claims have to be submitted with fuel samples and the old fuel filters to verify that red diesel hasn't been used.

Surely they wouldn't go to all this bother if red and white were the same stuff.

You'd need to add expensive additives to this so called "Red ultra low sulphur diesel", to make it anywhere near the same as white ULSD, but in ensuring customers add the correct amount every time is impossible. This is why most engine manufacturers who have engines that comply with Tier4 emissions advise against using it. It'd also negate any financial benefits from using red diesel.
I'm not sure what spec the marine engines are that you use, but the machine engines that have stopped using red are very low emission in order to comply with Boris Johnsons strict guidelines in central London and in underground tunnel applications such as CrossRail.